/* We use the reflection formula
Gamma(1+t) Gamma(1-t) = - Pi t / sin(Pi (1 + t))
in order to treat the case x <= 1,
i.e. with x = 1-t, then Gamma(x) = -Pi*(1-x)/sin(Pi*(2-x))/GAMMA(2-x)
*/
int
mpfr_gamma (mpfr_ptr gamma, mpfr_srcptr x, mpfr_rnd_t rnd_mode)
{
mpfr_t xp, GammaTrial, tmp, tmp2;
mpz_t fact;
mpfr_prec_t realprec;
int compared, is_integer;
int inex = 0; /* 0 means: result gamma not set yet */
MPFR_GROUP_DECL (group);
MPFR_SAVE_EXPO_DECL (expo);
MPFR_ZIV_DECL (loop);
MPFR_LOG_FUNC
(("x[%Pu]=%.*Rg rnd=%d", mpfr_get_prec (x), mpfr_log_prec, x, rnd_mode),
("gamma[%Pu]=%.*Rg inexact=%d",
mpfr_get_prec (gamma), mpfr_log_prec, gamma, inex));
/* Trivial cases */
if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (x)))
{
if (MPFR_IS_NAN (x))
{
MPFR_SET_NAN (gamma);
MPFR_RET_NAN;
}
else if (MPFR_IS_INF (x))
{
if (MPFR_IS_NEG (x))
{
MPFR_SET_NAN (gamma);
MPFR_RET_NAN;
}
else
{
MPFR_SET_INF (gamma);
MPFR_SET_POS (gamma);
MPFR_RET (0); /* exact */
}
}
else /* x is zero */
{
MPFR_ASSERTD(MPFR_IS_ZERO(x));
MPFR_SET_INF(gamma);
MPFR_SET_SAME_SIGN(gamma, x);
MPFR_SET_DIVBY0 ();
MPFR_RET (0); /* exact */
}
}
/* Check for tiny arguments, where gamma(x) ~ 1/x - euler + ....
We know from "Bound on Runs of Zeros and Ones for Algebraic Functions",
Proceedings of Arith15, T. Lang and J.-M. Muller, 2001, that the maximal
number of consecutive zeroes or ones after the round bit is n-1 for an
input of n bits. But we need a more precise lower bound. Assume x has
n bits, and 1/x is near a floating-point number y of n+1 bits. We can
write x = X*2^e, y = Y/2^f with X, Y integers of n and n+1 bits.
Thus X*Y^2^(e-f) is near from 1, i.e., X*Y is near from 2^(f-e).
Two cases can happen:
(i) either X*Y is exactly 2^(f-e), but this can happen only if X and Y
are themselves powers of two, i.e., x is a power of two;
(ii) or X*Y is at distance at least one from 2^(f-e), thus
|xy-1| >= 2^(e-f), or |y-1/x| >= 2^(e-f)/x = 2^(-f)/X >= 2^(-f-n).
Since ufp(y) = 2^(n-f) [ufp = unit in first place], this means
that the distance |y-1/x| >= 2^(-2n) ufp(y).
Now assuming |gamma(x)-1/x| <= 1, which is true for x <= 1,
if 2^(-2n) ufp(y) >= 2, the error is at most 2^(-2n-1) ufp(y),
and round(1/x) with precision >= 2n+2 gives the correct result.
If x < 2^E, then y > 2^(-E), thus ufp(y) > 2^(-E-1).
A sufficient condition is thus EXP(x) + 2 <= -2 MAX(PREC(x),PREC(Y)).
*/
if (MPFR_GET_EXP (x) + 2
<= -2 * (mpfr_exp_t) MAX(MPFR_PREC(x), MPFR_PREC(gamma)))
{
int sign = MPFR_SIGN (x); /* retrieve sign before possible override */
int special;
MPFR_BLOCK_DECL (flags);
MPFR_SAVE_EXPO_MARK (expo);
/* for overflow cases, see below; this needs to be done
before x possibly gets overridden. */
special =
MPFR_GET_EXP (x) == 1 - MPFR_EMAX_MAX &&
MPFR_IS_POS_SIGN (sign) &&
MPFR_IS_LIKE_RNDD (rnd_mode, sign) &&
mpfr_powerof2_raw (x);
MPFR_BLOCK (flags, inex = mpfr_ui_div (gamma, 1, x, rnd_mode));
if (inex == 0) /* x is a power of two */
{
/* return RND(1/x - euler) = RND(+/- 2^k - eps) with eps > 0 */
if (rnd_mode == MPFR_RNDN || MPFR_IS_LIKE_RNDU (rnd_mode, sign))
inex = 1;
else
{
mpfr_nextbelow (gamma);
inex = -1;
}
}
else if (MPFR_UNLIKELY (MPFR_OVERFLOW (flags)))
{
/* Overflow in the division 1/x. This is a real overflow, except
in RNDZ or RNDD when 1/x = 2^emax, i.e. x = 2^(-emax): due to
the "- euler", the rounded value in unbounded exponent range
is 0.111...11 * 2^emax (not an overflow). */
if (!special)
MPFR_SAVE_EXPO_UPDATE_FLAGS (expo, flags);
}
MPFR_SAVE_EXPO_FREE (expo);
/* Note: an overflow is possible with an infinite result;
in this case, the overflow flag will automatically be
restored by mpfr_check_range. */
return mpfr_check_range (gamma, inex, rnd_mode);
}
is_integer = mpfr_integer_p (x);
/* gamma(x) for x a negative integer gives NaN */
if (is_integer && MPFR_IS_NEG(x))
{
MPFR_SET_NAN (gamma);
MPFR_RET_NAN;
}
compared = mpfr_cmp_ui (x, 1);
if (compared == 0)
return mpfr_set_ui (gamma, 1, rnd_mode);
/* if x is an integer that fits into an unsigned long, use mpfr_fac_ui
if argument is not too large.
If precision is p, fac_ui costs O(u*p), whereas gamma costs O(p*M(p)),
so for u <= M(p), fac_ui should be faster.
We approximate here M(p) by p*log(p)^2, which is not a bad guess.
Warning: since the generic code does not handle exact cases,
we want all cases where gamma(x) is exact to be treated here.
*/
if (is_integer && mpfr_fits_ulong_p (x, MPFR_RNDN))
{
unsigned long int u;
mpfr_prec_t p = MPFR_PREC(gamma);
u = mpfr_get_ui (x, MPFR_RNDN);
if (u < 44787929UL && bits_fac (u - 1) <= p + (rnd_mode == MPFR_RNDN))
/* bits_fac: lower bound on the number of bits of m,
where gamma(x) = (u-1)! = m*2^e with m odd. */
return mpfr_fac_ui (gamma, u - 1, rnd_mode);
/* if bits_fac(...) > p (resp. p+1 for rounding to nearest),
then gamma(x) cannot be exact in precision p (resp. p+1).
FIXME: remove the test u < 44787929UL after changing bits_fac
to return a mpz_t or mpfr_t. */
}
MPFR_SAVE_EXPO_MARK (expo);
/* check for overflow: according to (6.1.37) in Abramowitz & Stegun,
gamma(x) >= exp(-x) * x^(x-1/2) * sqrt(2*Pi)
>= 2 * (x/e)^x / x for x >= 1 */
if (compared > 0)
{
mpfr_t yp;
mpfr_exp_t expxp;
MPFR_BLOCK_DECL (flags);
/* quick test for the default exponent range */
if (mpfr_get_emax () >= 1073741823UL && MPFR_GET_EXP(x) <= 25)
{
MPFR_SAVE_EXPO_FREE (expo);
return mpfr_gamma_aux (gamma, x, rnd_mode);
}
/* 1/e rounded down to 53 bits */
#define EXPM1_STR "0.010111100010110101011000110110001011001110111100111"
mpfr_init2 (xp, 53);
mpfr_init2 (yp, 53);
mpfr_set_str_binary (xp, EXPM1_STR);
mpfr_mul (xp, x, xp, MPFR_RNDZ);
mpfr_sub_ui (yp, x, 2, MPFR_RNDZ);
mpfr_pow (xp, xp, yp, MPFR_RNDZ); /* (x/e)^(x-2) */
mpfr_set_str_binary (yp, EXPM1_STR);
mpfr_mul (xp, xp, yp, MPFR_RNDZ); /* x^(x-2) / e^(x-1) */
mpfr_mul (xp, xp, yp, MPFR_RNDZ); /* x^(x-2) / e^x */
mpfr_mul (xp, xp, x, MPFR_RNDZ); /* lower bound on x^(x-1) / e^x */
MPFR_BLOCK (flags, mpfr_mul_2ui (xp, xp, 1, MPFR_RNDZ));
expxp = MPFR_GET_EXP (xp);
mpfr_clear (xp);
mpfr_clear (yp);
MPFR_SAVE_EXPO_FREE (expo);
return MPFR_OVERFLOW (flags) || expxp > __gmpfr_emax ?
mpfr_overflow (gamma, rnd_mode, 1) :
mpfr_gamma_aux (gamma, x, rnd_mode);
}
/* now compared < 0 */
/* check for underflow: for x < 1,
gamma(x) = Pi*(x-1)/sin(Pi*(2-x))/gamma(2-x).
Since gamma(2-x) >= 2 * ((2-x)/e)^(2-x) / (2-x), we have
|gamma(x)| <= Pi*(1-x)*(2-x)/2/((2-x)/e)^(2-x) / |sin(Pi*(2-x))|
<= 12 * ((2-x)/e)^x / |sin(Pi*(2-x))|.
To avoid an underflow in ((2-x)/e)^x, we compute the logarithm.
*/
if (MPFR_IS_NEG(x))
{
int underflow = 0, sgn, ck;
mpfr_prec_t w;
mpfr_init2 (xp, 53);
mpfr_init2 (tmp, 53);
mpfr_init2 (tmp2, 53);
/* we want an upper bound for x * [log(2-x)-1].
since x < 0, we need a lower bound on log(2-x) */
mpfr_ui_sub (xp, 2, x, MPFR_RNDD);
mpfr_log (xp, xp, MPFR_RNDD);
mpfr_sub_ui (xp, xp, 1, MPFR_RNDD);
mpfr_mul (xp, xp, x, MPFR_RNDU);
/* we need an upper bound on 1/|sin(Pi*(2-x))|,
thus a lower bound on |sin(Pi*(2-x))|.
If 2-x is exact, then the error of Pi*(2-x) is (1+u)^2 with u = 2^(-p)
thus the error on sin(Pi*(2-x)) is less than 1/2ulp + 3Pi(2-x)u,
assuming u <= 1, thus <= u + 3Pi(2-x)u */
w = mpfr_gamma_2_minus_x_exact (x); /* 2-x is exact for prec >= w */
w += 17; /* to get tmp2 small enough */
mpfr_set_prec (tmp, w);
mpfr_set_prec (tmp2, w);
MPFR_DBGRES (ck = mpfr_ui_sub (tmp, 2, x, MPFR_RNDN));
MPFR_ASSERTD (ck == 0); /* tmp = 2-x exactly */
mpfr_const_pi (tmp2, MPFR_RNDN);
mpfr_mul (tmp2, tmp2, tmp, MPFR_RNDN); /* Pi*(2-x) */
mpfr_sin (tmp, tmp2, MPFR_RNDN); /* sin(Pi*(2-x)) */
sgn = mpfr_sgn (tmp);
mpfr_abs (tmp, tmp, MPFR_RNDN);
mpfr_mul_ui (tmp2, tmp2, 3, MPFR_RNDU); /* 3Pi(2-x) */
mpfr_add_ui (tmp2, tmp2, 1, MPFR_RNDU); /* 3Pi(2-x)+1 */
mpfr_div_2ui (tmp2, tmp2, mpfr_get_prec (tmp), MPFR_RNDU);
/* if tmp2<|tmp|, we get a lower bound */
if (mpfr_cmp (tmp2, tmp) < 0)
{
mpfr_sub (tmp, tmp, tmp2, MPFR_RNDZ); /* low bnd on |sin(Pi*(2-x))| */
mpfr_ui_div (tmp, 12, tmp, MPFR_RNDU); /* upper bound */
mpfr_log2 (tmp, tmp, MPFR_RNDU);
mpfr_add (xp, tmp, xp, MPFR_RNDU);
/* The assert below checks that expo.saved_emin - 2 always
fits in a long. FIXME if we want to allow mpfr_exp_t to
be a long long, for instance. */
MPFR_ASSERTN (MPFR_EMIN_MIN - 2 >= LONG_MIN);
underflow = mpfr_cmp_si (xp, expo.saved_emin - 2) <= 0;
}
mpfr_clear (xp);
mpfr_clear (tmp);
mpfr_clear (tmp2);
if (underflow) /* the sign is the opposite of that of sin(Pi*(2-x)) */
{
MPFR_SAVE_EXPO_FREE (expo);
return mpfr_underflow (gamma, (rnd_mode == MPFR_RNDN) ? MPFR_RNDZ : rnd_mode, -sgn);
}
}
realprec = MPFR_PREC (gamma);
/* we want both 1-x and 2-x to be exact */
{
mpfr_prec_t w;
w = mpfr_gamma_1_minus_x_exact (x);
if (realprec < w)
realprec = w;
w = mpfr_gamma_2_minus_x_exact (x);
if (realprec < w)
realprec = w;
}
realprec = realprec + MPFR_INT_CEIL_LOG2 (realprec) + 20;
MPFR_ASSERTD(realprec >= 5);
MPFR_GROUP_INIT_4 (group, realprec + MPFR_INT_CEIL_LOG2 (realprec) + 20,
xp, tmp, tmp2, GammaTrial);
mpz_init (fact);
MPFR_ZIV_INIT (loop, realprec);
for (;;)
{
mpfr_exp_t err_g;
int ck;
MPFR_GROUP_REPREC_4 (group, realprec, xp, tmp, tmp2, GammaTrial);
/* reflection formula: gamma(x) = Pi*(x-1)/sin(Pi*(2-x))/gamma(2-x) */
ck = mpfr_ui_sub (xp, 2, x, MPFR_RNDN); /* 2-x, exact */
MPFR_ASSERTD(ck == 0); (void) ck; /* use ck to avoid a warning */
mpfr_gamma (tmp, xp, MPFR_RNDN); /* gamma(2-x), error (1+u) */
mpfr_const_pi (tmp2, MPFR_RNDN); /* Pi, error (1+u) */
mpfr_mul (GammaTrial, tmp2, xp, MPFR_RNDN); /* Pi*(2-x), error (1+u)^2 */
err_g = MPFR_GET_EXP(GammaTrial);
mpfr_sin (GammaTrial, GammaTrial, MPFR_RNDN); /* sin(Pi*(2-x)) */
/* If tmp is +Inf, we compute exp(lngamma(x)). */
if (mpfr_inf_p (tmp))
{
inex = mpfr_explgamma (gamma, x, &expo, tmp, tmp2, rnd_mode);
if (inex)
goto end;
else
goto ziv_next;
}
err_g = err_g + 1 - MPFR_GET_EXP(GammaTrial);
/* let g0 the true value of Pi*(2-x), g the computed value.
We have g = g0 + h with |h| <= |(1+u^2)-1|*g.
Thus sin(g) = sin(g0) + h' with |h'| <= |(1+u^2)-1|*g.
The relative error is thus bounded by |(1+u^2)-1|*g/sin(g)
<= |(1+u^2)-1|*2^err_g. <= 2.25*u*2^err_g for |u|<=1/4.
With the rounding error, this gives (0.5 + 2.25*2^err_g)*u. */
ck = mpfr_sub_ui (xp, x, 1, MPFR_RNDN); /* x-1, exact */
MPFR_ASSERTD(ck == 0); (void) ck; /* use ck to avoid a warning */
mpfr_mul (xp, tmp2, xp, MPFR_RNDN); /* Pi*(x-1), error (1+u)^2 */
mpfr_mul (GammaTrial, GammaTrial, tmp, MPFR_RNDN);
/* [1 + (0.5 + 2.25*2^err_g)*u]*(1+u)^2 = 1 + (2.5 + 2.25*2^err_g)*u
+ (0.5 + 2.25*2^err_g)*u*(2u+u^2) + u^2.
For err_g <= realprec-2, we have (0.5 + 2.25*2^err_g)*u <=
0.5*u + 2.25/4 <= 0.6875 and u^2 <= u/4, thus
(0.5 + 2.25*2^err_g)*u*(2u+u^2) + u^2 <= 0.6875*(2u+u/4) + u/4
<= 1.8*u, thus the rel. error is bounded by (4.5 + 2.25*2^err_g)*u. */
mpfr_div (GammaTrial, xp, GammaTrial, MPFR_RNDN);
/* the error is of the form (1+u)^3/[1 + (4.5 + 2.25*2^err_g)*u].
For realprec >= 5 and err_g <= realprec-2, [(4.5 + 2.25*2^err_g)*u]^2
<= 0.71, and for |y|<=0.71, 1/(1-y) can be written 1+a*y with a<=4.
(1+u)^3 * (1+4*(4.5 + 2.25*2^err_g)*u)
= 1 + (21 + 9*2^err_g)*u + (57+27*2^err_g)*u^2 + (55+27*2^err_g)*u^3
+ (18+9*2^err_g)*u^4
<= 1 + (21 + 9*2^err_g)*u + (57+27*2^err_g)*u^2 + (56+28*2^err_g)*u^3
<= 1 + (21 + 9*2^err_g)*u + (59+28*2^err_g)*u^2
<= 1 + (23 + 10*2^err_g)*u.
The final error is thus bounded by (23 + 10*2^err_g) ulps,
which is <= 2^6 for err_g<=2, and <= 2^(err_g+4) for err_g >= 2. */
err_g = (err_g <= 2) ? 6 : err_g + 4;
if (MPFR_LIKELY (MPFR_CAN_ROUND (GammaTrial, realprec - err_g,
MPFR_PREC(gamma), rnd_mode)))
break;
ziv_next:
MPFR_ZIV_NEXT (loop, realprec);
}
end:
MPFR_ZIV_FREE (loop);
if (inex == 0)
inex = mpfr_set (gamma, GammaTrial, rnd_mode);
MPFR_GROUP_CLEAR (group);
mpz_clear (fact);
MPFR_SAVE_EXPO_FREE (expo);
return mpfr_check_range (gamma, inex, rnd_mode);
}
int
mpfr_zeta (mpfr_t z, mpfr_srcptr s, mp_rnd_t rnd_mode)
{
mpfr_t z_pre, s1, y, p;
double sd, eps, m1, c;
long add;
mp_prec_t precz, prec1, precs, precs1;
int inex;
MPFR_GROUP_DECL (group);
MPFR_ZIV_DECL (loop);
MPFR_SAVE_EXPO_DECL (expo);
MPFR_LOG_FUNC (("s[%#R]=%R rnd=%d", s, s, rnd_mode),
("z[%#R]=%R inexact=%d", z, z, inex));
/* Zero, Nan or Inf ? */
if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (s)))
{
if (MPFR_IS_NAN (s))
{
MPFR_SET_NAN (z);
MPFR_RET_NAN;
}
else if (MPFR_IS_INF (s))
{
if (MPFR_IS_POS (s))
return mpfr_set_ui (z, 1, GMP_RNDN); /* Zeta(+Inf) = 1 */
MPFR_SET_NAN (z); /* Zeta(-Inf) = NaN */
MPFR_RET_NAN;
}
else /* s iz zero */
{
MPFR_ASSERTD (MPFR_IS_ZERO (s));
mpfr_set_ui (z, 1, rnd_mode);
mpfr_div_2ui (z, z, 1, rnd_mode);
MPFR_CHANGE_SIGN (z);
MPFR_RET (0);
}
}
/* s is neither Nan, nor Inf, nor Zero */
/* check tiny s: we have zeta(s) = -1/2 - 1/2 log(2 Pi) s + ... around s=0,
and for |s| <= 0.074, we have |zeta(s) + 1/2| <= |s|.
Thus if |s| <= 1/4*ulp(1/2), we can deduce the correct rounding
(the 1/4 covers the case where |zeta(s)| < 1/2 and rounding to nearest).
A sufficient condition is that EXP(s) + 1 < -PREC(z). */
if (MPFR_EXP(s) + 1 < - (mp_exp_t) MPFR_PREC(z))
{
int signs = MPFR_SIGN(s);
mpfr_set_si_2exp (z, -1, -1, rnd_mode); /* -1/2 */
if ((rnd_mode == GMP_RNDU || rnd_mode == GMP_RNDZ) && signs < 0)
{
mpfr_nextabove (z); /* z = -1/2 + epsilon */
inex = 1;
}
else if (rnd_mode == GMP_RNDD && signs > 0)
{
mpfr_nextbelow (z); /* z = -1/2 - epsilon */
inex = -1;
}
else
{
if (rnd_mode == GMP_RNDU) /* s > 0: z = -1/2 */
inex = 1;
else if (rnd_mode == GMP_RNDD)
inex = -1; /* s < 0: z = -1/2 */
else /* (GMP_RNDZ and s > 0) or GMP_RNDN: z = -1/2 */
inex = (signs > 0) ? 1 : -1;
}
return mpfr_check_range (z, inex, rnd_mode);
}
/* Check for case s= -2n */
if (MPFR_IS_NEG (s))
{
mpfr_t tmp;
tmp[0] = *s;
MPFR_EXP (tmp) = MPFR_EXP (s) - 1;
if (mpfr_integer_p (tmp))
{
MPFR_SET_ZERO (z);
MPFR_SET_POS (z);
MPFR_RET (0);
}
}
MPFR_SAVE_EXPO_MARK (expo);
/* Compute Zeta */
if (MPFR_IS_POS (s) && MPFR_GET_EXP (s) >= 0) /* Case s >= 1/2 */
inex = mpfr_zeta_pos (z, s, rnd_mode);
else /* use reflection formula
zeta(s) = 2^s*Pi^(s-1)*sin(Pi*s/2)*gamma(1-s)*zeta(1-s) */
{
precz = MPFR_PREC (z);
precs = MPFR_PREC (s);
/* Precision precs1 needed to represent 1 - s, and s + 2,
without any truncation */
precs1 = precs + 2 + MAX (0, - MPFR_GET_EXP (s));
sd = mpfr_get_d (s, GMP_RNDN) - 1.0;
if (sd < 0.0)
sd = -sd; /* now sd = abs(s-1.0) */
/* Precision prec1 is the precision on elementary computations;
it ensures a final precision prec1 - add for zeta(s) */
/* eps = pow (2.0, - (double) precz - 14.0); */
eps = __gmpfr_ceil_exp2 (- (double) precz - 14.0);
m1 = 1.0 + MAX(1.0 / eps, 2.0 * sd) * (1.0 + eps);
c = (1.0 + eps) * (1.0 + eps * MAX(8.0, m1));
/* add = 1 + floor(log(c*c*c*(13 + m1))/log(2)); */
add = __gmpfr_ceil_log2 (c * c * c * (13.0 + m1));
prec1 = precz + add;
prec1 = MAX (prec1, precs1) + 10;
MPFR_GROUP_INIT_4 (group, prec1, z_pre, s1, y, p);
MPFR_ZIV_INIT (loop, prec1);
for (;;)
{
mpfr_sub (s1, __gmpfr_one, s, GMP_RNDN);/* s1 = 1-s */
mpfr_zeta_pos (z_pre, s1, GMP_RNDN); /* zeta(1-s) */
mpfr_gamma (y, s1, GMP_RNDN); /* gamma(1-s) */
if (MPFR_IS_INF (y)) /* Zeta(s) < 0 for -4k-2 < s < -4k,
Zeta(s) > 0 for -4k < s < -4k+2 */
{
MPFR_SET_INF (z_pre);
mpfr_div_2ui (s1, s, 2, GMP_RNDN); /* s/4, exact */
mpfr_frac (s1, s1, GMP_RNDN); /* exact, -1 < s1 < 0 */
if (mpfr_cmp_si_2exp (s1, -1, -1) > 0)
MPFR_SET_NEG (z_pre);
else
MPFR_SET_POS (z_pre);
break;
}
mpfr_mul (z_pre, z_pre, y, GMP_RNDN); /* gamma(1-s)*zeta(1-s) */
mpfr_const_pi (p, GMP_RNDD);
mpfr_mul (y, s, p, GMP_RNDN);
mpfr_div_2ui (y, y, 1, GMP_RNDN); /* s*Pi/2 */
mpfr_sin (y, y, GMP_RNDN); /* sin(Pi*s/2) */
mpfr_mul (z_pre, z_pre, y, GMP_RNDN);
mpfr_mul_2ui (y, p, 1, GMP_RNDN); /* 2*Pi */
mpfr_neg (s1, s1, GMP_RNDN); /* s-1 */
mpfr_pow (y, y, s1, GMP_RNDN); /* (2*Pi)^(s-1) */
mpfr_mul (z_pre, z_pre, y, GMP_RNDN);
mpfr_mul_2ui (z_pre, z_pre, 1, GMP_RNDN);
if (MPFR_LIKELY (MPFR_CAN_ROUND (z_pre, prec1 - add, precz,
rnd_mode)))
break;
MPFR_ZIV_NEXT (loop, prec1);
MPFR_GROUP_REPREC_4 (group, prec1, z_pre, s1, y, p);
}
MPFR_ZIV_FREE (loop);
inex = mpfr_set (z, z_pre, rnd_mode);
MPFR_GROUP_CLEAR (group);
}
MPFR_SAVE_EXPO_FREE (expo);
return mpfr_check_range (z, inex, rnd_mode);
}
/* evaluates erf(x) using the expansion at x=0:
erf(x) = 2/sqrt(Pi) * sum((-1)^k*x^(2k+1)/k!/(2k+1), k=0..infinity)
Assumes x is neither NaN nor infinite nor zero.
Assumes also that e*x^2 <= n (target precision).
*/
static int
mpfr_erf_0 (mpfr_ptr res, mpfr_srcptr x, double xf2, mpfr_rnd_t rnd_mode)
{
mpfr_prec_t n, m;
mpfr_exp_t nuk, sigmak;
double tauk;
mpfr_t y, s, t, u;
unsigned int k;
int log2tauk;
int inex;
MPFR_GROUP_DECL (group);
MPFR_ZIV_DECL (loop);
n = MPFR_PREC (res); /* target precision */
/* initial working precision */
m = n + (mpfr_prec_t) (xf2 / LOG2) + 8 + MPFR_INT_CEIL_LOG2 (n);
MPFR_GROUP_INIT_4(group, m, y, s, t, u);
MPFR_ZIV_INIT (loop, m);
for (;;)
{
mpfr_mul (y, x, x, MPFR_RNDU); /* err <= 1 ulp */
mpfr_set_ui (s, 1, MPFR_RNDN);
mpfr_set_ui (t, 1, MPFR_RNDN);
tauk = 0.0;
for (k = 1; ; k++)
{
mpfr_mul (t, y, t, MPFR_RNDU);
mpfr_div_ui (t, t, k, MPFR_RNDU);
mpfr_div_ui (u, t, 2 * k + 1, MPFR_RNDU);
sigmak = MPFR_GET_EXP (s);
if (k % 2)
mpfr_sub (s, s, u, MPFR_RNDN);
else
mpfr_add (s, s, u, MPFR_RNDN);
sigmak -= MPFR_GET_EXP(s);
nuk = MPFR_GET_EXP(u) - MPFR_GET_EXP(s);
if ((nuk < - (mpfr_exp_t) m) && ((double) k >= xf2))
break;
/* tauk <- 1/2 + tauk * 2^sigmak + (1+8k)*2^nuk */
tauk = 0.5 + mul_2exp (tauk, sigmak)
+ mul_2exp (1.0 + 8.0 * (double) k, nuk);
}
mpfr_mul (s, x, s, MPFR_RNDU);
MPFR_SET_EXP (s, MPFR_GET_EXP (s) + 1);
mpfr_const_pi (t, MPFR_RNDZ);
mpfr_sqrt (t, t, MPFR_RNDZ);
mpfr_div (s, s, t, MPFR_RNDN);
tauk = 4.0 * tauk + 11.0; /* final ulp-error on s */
log2tauk = __gmpfr_ceil_log2 (tauk);
if (MPFR_LIKELY (MPFR_CAN_ROUND (s, m - log2tauk, n, rnd_mode)))
break;
/* Actualisation of the precision */
MPFR_ZIV_NEXT (loop, m);
MPFR_GROUP_REPREC_4 (group, m, y, s, t, u);
}
MPFR_ZIV_FREE (loop);
inex = mpfr_set (res, s, rnd_mode);
MPFR_GROUP_CLEAR (group);
return inex;
}
/* Input: s - a floating-point number >= 1/2.
rnd_mode - a rounding mode.
Assumes s is neither NaN nor Infinite.
Output: z - Zeta(s) rounded to the precision of z with direction rnd_mode
*/
static int
mpfr_zeta_pos (mpfr_t z, mpfr_srcptr s, mp_rnd_t rnd_mode)
{
mpfr_t b, c, z_pre, f, s1;
double beta, sd, dnep;
mpfr_t *tc1;
mp_prec_t precz, precs, d, dint;
int p, n, l, add;
int inex;
MPFR_GROUP_DECL (group);
MPFR_ZIV_DECL (loop);
MPFR_ASSERTD (MPFR_IS_POS (s) && MPFR_GET_EXP (s) >= 0);
precz = MPFR_PREC (z);
precs = MPFR_PREC (s);
/* Zeta(x) = 1+1/2^x+1/3^x+1/4^x+1/5^x+O(1/6^x)
so with 2^(EXP(x)-1) <= x < 2^EXP(x)
So for x > 2^3, k^x > k^8, so 2/k^x < 2/k^8
Zeta(x) = 1 + 1/2^x*(1+(2/3)^x+(2/4)^x+...)
= 1 + 1/2^x*(1+sum((2/k)^x,k=3..infinity))
<= 1 + 1/2^x*(1+sum((2/k)^8,k=3..infinity))
And sum((2/k)^8,k=3..infinity) = -257+128*Pi^8/4725 ~= 0.0438035
So Zeta(x) <= 1 + 1/2^x*2 for x >= 8
The error is < 2^(-x+1) <= 2^(-2^(EXP(x)-1)+1) */
if (MPFR_GET_EXP (s) > 3)
{
mp_exp_t err;
err = MPFR_GET_EXP (s) - 1;
if (err > (mp_exp_t) (sizeof (mp_exp_t)*CHAR_BIT-2))
err = MPFR_EMAX_MAX;
else
err = ((mp_exp_t)1) << err;
err = 1 - (-err+1); /* GET_EXP(one) - (-err+1) = err :) */
MPFR_FAST_COMPUTE_IF_SMALL_INPUT (z, __gmpfr_one, err, 0, 1,
rnd_mode, {});
}
d = precz + MPFR_INT_CEIL_LOG2(precz) + 10;
/* we want that s1 = s-1 is exact, i.e. we should have PREC(s1) >= EXP(s) */
dint = (mpfr_uexp_t) MPFR_GET_EXP (s);
mpfr_init2 (s1, MAX (precs, dint));
inex = mpfr_sub (s1, s, __gmpfr_one, GMP_RNDN);
MPFR_ASSERTD (inex == 0);
/* case s=1 */
if (MPFR_IS_ZERO (s1))
{
MPFR_SET_INF (z);
MPFR_SET_POS (z);
MPFR_ASSERTD (inex == 0);
goto clear_and_return;
}
MPFR_GROUP_INIT_4 (group, MPFR_PREC_MIN, b, c, z_pre, f);
MPFR_ZIV_INIT (loop, d);
for (;;)
{
/* Principal loop: we compute, in z_pre,
an approximation of Zeta(s), that we send to can_round */
if (MPFR_GET_EXP (s1) <= -(mp_exp_t) ((mpfr_prec_t) (d-3)/2))
/* Branch 1: when s-1 is very small, one
uses the approximation Zeta(s)=1/(s-1)+gamma,
where gamma is Euler's constant */
{
dint = MAX (d + 3, precs);
MPFR_TRACE (printf ("branch 1\ninternal precision=%d\n", dint));
MPFR_GROUP_REPREC_4 (group, dint, b, c, z_pre, f);
mpfr_div (z_pre, __gmpfr_one, s1, GMP_RNDN);
mpfr_const_euler (f, GMP_RNDN);
mpfr_add (z_pre, z_pre, f, GMP_RNDN);
}
else /* Branch 2 */
{
size_t size;
MPFR_TRACE (printf ("branch 2\n"));
/* Computation of parameters n, p and working precision */
dnep = (double) d * LOG2;
sd = mpfr_get_d (s, GMP_RNDN);
/* beta = dnep + 0.61 + sd * log (6.2832 / sd);
but a larger value is ok */
#define LOG6dot2832 1.83787940484160805532
beta = dnep + 0.61 + sd * (LOG6dot2832 - LOG2 *
__gmpfr_floor_log2 (sd));
if (beta <= 0.0)
{
p = 0;
/* n = 1 + (int) (exp ((dnep - LOG2) / sd)); */
n = 1 + (int) __gmpfr_ceil_exp2 ((d - 1.0) / sd);
}
else
{
p = 1 + (int) beta / 2;
n = 1 + (int) ((sd + 2.0 * (double) p - 1.0) / 6.2832);
}
MPFR_TRACE (printf ("\nn=%d\np=%d\n",n,p));
/* add = 4 + floor(1.5 * log(d) / log (2)).
We should have add >= 10, which is always fulfilled since
d = precz + 11 >= 12, thus ceil(log2(d)) >= 4 */
add = 4 + (3 * MPFR_INT_CEIL_LOG2 (d)) / 2;
MPFR_ASSERTD(add >= 10);
dint = d + add;
if (dint < precs)
dint = precs;
MPFR_TRACE (printf("internal precision=%d\n",dint));
size = (p + 1) * sizeof(mpfr_t);
tc1 = (mpfr_t*) (*__gmp_allocate_func) (size);
for (l=1; l<=p; l++)
mpfr_init2 (tc1[l], dint);
MPFR_GROUP_REPREC_4 (group, dint, b, c, z_pre, f);
MPFR_TRACE (printf ("precision of z =%d\n", precz));
/* Computation of the coefficients c_k */
mpfr_zeta_c (p, tc1);
/* Computation of the 3 parts of the fonction Zeta. */
mpfr_zeta_part_a (z_pre, s, n);
mpfr_zeta_part_b (b, s, n, p, tc1);
/* s1 = s-1 is already computed above */
mpfr_div (c, __gmpfr_one, s1, GMP_RNDN);
mpfr_ui_pow (f, n, s1, GMP_RNDN);
mpfr_div (c, c, f, GMP_RNDN);
MPFR_TRACE (MPFR_DUMP (c));
mpfr_add (z_pre, z_pre, c, GMP_RNDN);
mpfr_add (z_pre, z_pre, b, GMP_RNDN);
for (l=1; l<=p; l++)
mpfr_clear (tc1[l]);
(*__gmp_free_func) (tc1, size);
/* End branch 2 */
}
MPFR_TRACE (MPFR_DUMP (z_pre));
if (MPFR_LIKELY (MPFR_CAN_ROUND (z_pre, d-3, precz, rnd_mode)))
break;
MPFR_ZIV_NEXT (loop, d);
}
MPFR_ZIV_FREE (loop);
inex = mpfr_set (z, z_pre, rnd_mode);
MPFR_GROUP_CLEAR (group);
clear_and_return:
mpfr_clear (s1);
return inex;
}
int
mpfr_zeta (mpfr_t z, mpfr_srcptr s, mpfr_rnd_t rnd_mode)
{
mpfr_t z_pre, s1, y, p;
long add;
mpfr_prec_t precz, prec1, precs, precs1;
int inex;
MPFR_GROUP_DECL (group);
MPFR_ZIV_DECL (loop);
MPFR_SAVE_EXPO_DECL (expo);
MPFR_LOG_FUNC (
("s[%Pu]=%.*Rg rnd=%d", mpfr_get_prec (s), mpfr_log_prec, s, rnd_mode),
("z[%Pu]=%.*Rg inexact=%d", mpfr_get_prec (z), mpfr_log_prec, z, inex));
/* Zero, Nan or Inf ? */
if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (s)))
{
if (MPFR_IS_NAN (s))
{
MPFR_SET_NAN (z);
MPFR_RET_NAN;
}
else if (MPFR_IS_INF (s))
{
if (MPFR_IS_POS (s))
return mpfr_set_ui (z, 1, MPFR_RNDN); /* Zeta(+Inf) = 1 */
MPFR_SET_NAN (z); /* Zeta(-Inf) = NaN */
MPFR_RET_NAN;
}
else /* s iz zero */
{
MPFR_ASSERTD (MPFR_IS_ZERO (s));
return mpfr_set_si_2exp (z, -1, -1, rnd_mode);
}
}
/* s is neither Nan, nor Inf, nor Zero */
/* check tiny s: we have zeta(s) = -1/2 - 1/2 log(2 Pi) s + ... around s=0,
and for |s| <= 2^(-4), we have |zeta(s) + 1/2| <= |s|.
EXP(s) + 1 < -PREC(z) is a sufficient condition to be able to round
correctly, for any PREC(z) >= 1 (see algorithms.tex for details). */
if (MPFR_GET_EXP (s) + 1 < - (mpfr_exp_t) MPFR_PREC(z))
{
int signs = MPFR_SIGN(s);
MPFR_SAVE_EXPO_MARK (expo);
mpfr_set_si_2exp (z, -1, -1, rnd_mode); /* -1/2 */
if (rnd_mode == MPFR_RNDA)
rnd_mode = MPFR_RNDD; /* the result is around -1/2, thus negative */
if ((rnd_mode == MPFR_RNDU || rnd_mode == MPFR_RNDZ) && signs < 0)
{
mpfr_nextabove (z); /* z = -1/2 + epsilon */
inex = 1;
}
else if (rnd_mode == MPFR_RNDD && signs > 0)
{
mpfr_nextbelow (z); /* z = -1/2 - epsilon */
inex = -1;
}
else
{
if (rnd_mode == MPFR_RNDU) /* s > 0: z = -1/2 */
inex = 1;
else if (rnd_mode == MPFR_RNDD)
inex = -1; /* s < 0: z = -1/2 */
else /* (MPFR_RNDZ and s > 0) or MPFR_RNDN: z = -1/2 */
inex = (signs > 0) ? 1 : -1;
}
MPFR_SAVE_EXPO_FREE (expo);
return mpfr_check_range (z, inex, rnd_mode);
}
/* Check for case s= -2n */
if (MPFR_IS_NEG (s))
{
mpfr_t tmp;
tmp[0] = *s;
MPFR_EXP (tmp) = MPFR_GET_EXP (s) - 1;
if (mpfr_integer_p (tmp))
{
MPFR_SET_ZERO (z);
MPFR_SET_POS (z);
MPFR_RET (0);
}
}
/* Check for case s=1 before changing the exponent range */
if (mpfr_cmp (s, __gmpfr_one) == 0)
{
MPFR_SET_INF (z);
MPFR_SET_POS (z);
MPFR_SET_DIVBY0 ();
MPFR_RET (0);
}
MPFR_SAVE_EXPO_MARK (expo);
/* Compute Zeta */
if (MPFR_IS_POS (s) && MPFR_GET_EXP (s) >= 0) /* Case s >= 1/2 */
inex = mpfr_zeta_pos (z, s, rnd_mode);
else /* use reflection formula
zeta(s) = 2^s*Pi^(s-1)*sin(Pi*s/2)*gamma(1-s)*zeta(1-s) */
{
int overflow = 0;
precz = MPFR_PREC (z);
precs = MPFR_PREC (s);
/* Precision precs1 needed to represent 1 - s, and s + 2,
without any truncation */
precs1 = precs + 2 + MAX (0, - MPFR_GET_EXP (s));
/* Precision prec1 is the precision on elementary computations;
it ensures a final precision prec1 - add for zeta(s) */
add = compute_add (s, precz);
prec1 = precz + add;
/* FIXME: To avoid that the working precision (prec1) depends on the
input precision, one would need to take into account the error made
when s1 is not exactly 1-s when computing zeta(s1) and gamma(s1)
below, and also in the case y=Inf (i.e. when gamma(s1) overflows).
Make sure that underflows do not occur in intermediate computations.
Due to the limited precision, they are probably not possible
in practice; add some MPFR_ASSERTN's to be sure that problems
do not remain undetected? */
prec1 = MAX (prec1, precs1) + 10;
MPFR_GROUP_INIT_4 (group, prec1, z_pre, s1, y, p);
MPFR_ZIV_INIT (loop, prec1);
for (;;)
{
mpfr_exp_t ey;
mpfr_t z_up;
mpfr_const_pi (p, MPFR_RNDD); /* p is Pi */
mpfr_sub (s1, __gmpfr_one, s, MPFR_RNDN); /* s1 = 1-s */
mpfr_gamma (y, s1, MPFR_RNDN); /* gamma(1-s) */
if (MPFR_IS_INF (y)) /* zeta(s) < 0 for -4k-2 < s < -4k,
zeta(s) > 0 for -4k < s < -4k+2 */
{
/* FIXME: An overflow in gamma(s1) does not imply that
zeta(s) will overflow. A solution:
1. Compute
log(|zeta(s)|/2) = (s-1)*log(2*pi) + lngamma(1-s)
+ log(abs(sin(Pi*s/2)) * zeta(1-s))
(possibly sharing computations with the normal case)
with a rather good accuracy (see (2)).
Memorize the sign of sin(...) for the final sign.
2. Take the exponential, ~= |zeta(s)|/2. If there is an
overflow, then this means an overflow on the final result
(due to the multiplication by 2, which has not been done
yet).
3. Ziv test.
4. Correct the sign from the sign of sin(...).
5. Round then multiply by 2. Here, an overflow in either
operation means a real overflow. */
mpfr_reflection_overflow (z_pre, s1, s, y, p, MPFR_RNDD);
/* z_pre is a lower bound of |zeta(s)|/2, thus if it overflows,
or has exponent emax, then |zeta(s)| overflows too. */
if (MPFR_IS_INF (z_pre) || MPFR_GET_EXP(z_pre) == __gmpfr_emax)
{ /* determine the sign of overflow */
mpfr_div_2ui (s1, s, 2, MPFR_RNDN); /* s/4, exact */
mpfr_frac (s1, s1, MPFR_RNDN); /* exact, -1 < s1 < 0 */
overflow = (mpfr_cmp_si_2exp (s1, -1, -1) > 0) ? -1 : 1;
break;
}
else /* EXP(z_pre) < __gmpfr_emax */
{
int ok = 0;
mpfr_t z_down;
mpfr_init2 (z_up, mpfr_get_prec (z_pre));
mpfr_reflection_overflow (z_up, s1, s, y, p, MPFR_RNDU);
/* if the lower approximation z_pre does not overflow, but
z_up does, we need more precision */
if (MPFR_IS_INF (z_up) || MPFR_GET_EXP(z_up) == __gmpfr_emax)
goto next_loop;
/* check if z_pre and z_up round to the same number */
mpfr_init2 (z_down, precz);
mpfr_set (z_down, z_pre, rnd_mode);
/* Note: it might be that EXP(z_down) = emax here, in that
case we will have overflow below when we multiply by 2 */
mpfr_prec_round (z_up, precz, rnd_mode);
ok = mpfr_cmp (z_down, z_up) == 0;
mpfr_clear (z_up);
mpfr_clear (z_down);
if (ok)
{
/* get correct sign and multiply by 2 */
mpfr_div_2ui (s1, s, 2, MPFR_RNDN); /* s/4, exact */
mpfr_frac (s1, s1, MPFR_RNDN); /* exact, -1 < s1 < 0 */
if (mpfr_cmp_si_2exp (s1, -1, -1) > 0)
mpfr_neg (z_pre, z_pre, rnd_mode);
mpfr_mul_2ui (z_pre, z_pre, 1, rnd_mode);
break;
}
else
goto next_loop;
}
}
mpfr_zeta_pos (z_pre, s1, MPFR_RNDN); /* zeta(1-s) */
mpfr_mul (z_pre, z_pre, y, MPFR_RNDN); /* gamma(1-s)*zeta(1-s) */
/* multiply z_pre by 2^s*Pi^(s-1) where p=Pi, s1=1-s */
mpfr_mul_2ui (y, p, 1, MPFR_RNDN); /* 2*Pi */
mpfr_neg (s1, s1, MPFR_RNDN); /* s-1 */
mpfr_pow (y, y, s1, MPFR_RNDN); /* (2*Pi)^(s-1) */
mpfr_mul (z_pre, z_pre, y, MPFR_RNDN);
mpfr_mul_2ui (z_pre, z_pre, 1, MPFR_RNDN);
/* multiply z_pre by sin(Pi*s/2) */
mpfr_mul (y, s, p, MPFR_RNDN);
mpfr_div_2ui (p, y, 1, MPFR_RNDN); /* p = s*Pi/2 */
/* FIXME: sinpi will be available, we should replace the mpfr_sin
call below by mpfr_sinpi(s/2), where s/2 will be exact.
Can mpfr_sin underflow? Moreover, the code below should be
improved so that the "if" condition becomes unlikely, e.g.
by taking a slightly larger working precision. */
mpfr_sin (y, p, MPFR_RNDN); /* y = sin(Pi*s/2) */
ey = MPFR_GET_EXP (y);
if (ey < 0) /* take account of cancellation in sin(p) */
{
mpfr_t t;
MPFR_ASSERTN (- ey < MPFR_PREC_MAX - prec1);
mpfr_init2 (t, prec1 - ey);
mpfr_const_pi (t, MPFR_RNDD);
mpfr_mul (t, s, t, MPFR_RNDN);
mpfr_div_2ui (t, t, 1, MPFR_RNDN);
mpfr_sin (y, t, MPFR_RNDN);
mpfr_clear (t);
}
mpfr_mul (z_pre, z_pre, y, MPFR_RNDN);
if (MPFR_LIKELY (MPFR_CAN_ROUND (z_pre, prec1 - add, precz,
rnd_mode)))
break;
next_loop:
MPFR_ZIV_NEXT (loop, prec1);
MPFR_GROUP_REPREC_4 (group, prec1, z_pre, s1, y, p);
}
MPFR_ZIV_FREE (loop);
if (overflow != 0)
{
inex = mpfr_overflow (z, rnd_mode, overflow);
MPFR_SAVE_EXPO_UPDATE_FLAGS (expo, MPFR_FLAGS_OVERFLOW);
}
else
inex = mpfr_set (z, z_pre, rnd_mode);
MPFR_GROUP_CLEAR (group);
}
MPFR_SAVE_EXPO_FREE (expo);
return mpfr_check_range (z, inex, rnd_mode);
}