void mag_polylog_tail(mag_t u, const mag_t z, long sigma, ulong d, ulong N) { mag_t TN, UN, t; if (N < 2) { mag_inf(u); return; } mag_init(TN); mag_init(UN); mag_init(t); if (mag_cmp_2exp_si(z, 0) >= 0) { mag_inf(u); } else { /* Bound T(N) */ mag_pow_ui(TN, z, N); /* multiply by log(N)^d */ if (d > 0) { mag_log_ui(t, N); mag_pow_ui(t, t, d); mag_mul(TN, TN, t); } /* multiply by 1/k^s */ if (sigma > 0) { mag_set_ui_lower(t, N); mag_pow_ui_lower(t, t, sigma); mag_div(TN, TN, t); } else if (sigma < 0) { mag_set_ui(t, N); mag_pow_ui(t, t, -sigma); mag_mul(TN, TN, t); } /* Bound U(N) */ mag_set(UN, z); /* multiply by (1 + 1/N)**S */ if (sigma < 0) { mag_binpow_uiui(t, N, -sigma); mag_mul(UN, UN, t); } /* multiply by (1 + 1/(N log(N)))^d */ if (d > 0) { ulong nl; /* rounds down */ nl = mag_d_log_lower_bound(N) * N * (1 - 1e-13); mag_binpow_uiui(t, nl, d); mag_mul(UN, UN, t); } /* T(N) / (1 - U(N)) */ if (mag_cmp_2exp_si(UN, 0) >= 0) { mag_inf(u); } else { mag_one(t); mag_sub_lower(t, t, UN); mag_div(u, TN, t); } } mag_clear(TN); mag_clear(UN); mag_clear(t); }
slong hypgeom_bound(mag_t error, int r, slong A, slong B, slong K, const mag_t TK, const mag_t z, slong tol_2exp) { mag_t Tn, t, u, one, tol, num, den; slong n, m; mag_init(Tn); mag_init(t); mag_init(u); mag_init(one); mag_init(tol); mag_init(num); mag_init(den); mag_one(one); mag_set_ui_2exp_si(tol, UWORD(1), -tol_2exp); /* approximate number of needed terms */ n = hypgeom_estimate_terms(z, r, tol_2exp); /* required for 1 + O(1/k) part to be decreasing */ n = FLINT_MAX(n, K + 1); /* required for z^k / (k!)^r to be decreasing */ m = hypgeom_root_bound(z, r); n = FLINT_MAX(n, m); /* We now have |R(k)| <= G(k) where G(k) is monotonically decreasing, and can bound the tail using a geometric series as soon as soon as G(k) < 1. */ /* bound T(n-1) */ hypgeom_term_bound(Tn, TK, K, A, B, r, z, n-1); while (1) { /* bound R(n) */ mag_mul_ui(num, z, n); mag_mul_ui(num, num, n - B); mag_set_ui_lower(den, n - A); mag_mul_ui_lower(den, den, n - 2*B); if (r != 0) { mag_set_ui_lower(u, n); mag_pow_ui_lower(u, u, r); mag_mul_lower(den, den, u); } mag_div(t, num, den); /* multiply bound for T(n-1) by bound for R(n) to bound T(n) */ mag_mul(Tn, Tn, t); /* geometric series termination check */ /* u = max(1-t, 0), rounding down [lower bound] */ mag_sub_lower(u, one, t); if (!mag_is_zero(u)) { mag_div(u, Tn, u); if (mag_cmp(u, tol) < 0) { mag_set(error, u); break; } } /* move on to next term */ n++; } mag_clear(Tn); mag_clear(t); mag_clear(u); mag_clear(one); mag_clear(tol); mag_clear(num); mag_clear(den); return n; }