FuncResult change_peripheral_curves(
Triangulation *manifold,
CONST MatrixInt22 change_matrices[])
{
int i,
v,
f,
old_m,
old_l;
double old_m_coef, /* changed from int to double, JRW 2000/01/18 */
old_l_coef;
Tetrahedron *tet;
Cusp *cusp;
Complex old_Hm,
old_Hl;
/*
* First make sure all the change_matrices have determinant +1.
*/
for (i = 0; i < manifold->num_cusps; i++)
if (DET2(change_matrices[i]) != +1)
return func_bad_input;
/*
* The change_matrices for Klein bottle cusps must have zeros in the
* off-diagonal entries. (Nothing else makes sense topologically.)
*/
for (cusp = manifold->cusp_list_begin.next;
cusp != &manifold->cusp_list_end;
cusp = cusp->next)
if (cusp->topology == Klein_cusp)
for (i = 0; i < 2; i++)
if (change_matrices[cusp->index][i][!i] != 0)
uFatalError("change_peripheral_curves", "change_peripheral_curves");
/*
* Change the peripheral curves according to the change_matrices.
* As stated at the top of this file, the transformation rule is
*
* | new m | | | | old m |
* | | = | change_matrices[i] | | |
* | new l | | | | old l |
*/
for (tet = manifold->tet_list_begin.next;
tet != &manifold->tet_list_end;
tet = tet->next)
for (i = 0; i < 2; i++) /* which orientation */
for (v = 0; v < 4; v++) /* which vertex */
for (f = 0; f < 4; f++) /* which side */
{
old_m = tet->curve[M][i][v][f];
old_l = tet->curve[L][i][v][f];
tet->curve[M][i][v][f]
= change_matrices[tet->cusp[v]->index][0][0] * old_m
+ change_matrices[tet->cusp[v]->index][0][1] * old_l;
tet->curve[L][i][v][f]
= change_matrices[tet->cusp[v]->index][1][0] * old_m
+ change_matrices[tet->cusp[v]->index][1][1] * old_l;
}
/*
* Change the Dehn filling coefficients to reflect the new
* peripheral curves. That is, we want the Dehn filling curves
* to be topologically the same as before, even though the
* coefficients will be different because we changed the basis.
*
* To keep our thinking straight, let's imagine all peripheral
* curves -- old and new -- in terms of some arbitrary but
* fixed basis for pi_1(T^2) = Z + Z. We never actually compute
* such a basis, but it helps keep our thinking straight.
* Relative to this fixed basis, we have
*
* old m = (old m [0], old m [1])
* old l = (old l [0], old l [1])
* new m = (new m [0], new m [1])
* new l = (new l [0], new l [1])
*
* Note that these m's and l's are curves, not coefficients!
* They are elements of pi_1(T^2) = Z + Z.
*
* We can then rewrite the above transformation rule, with
* each peripheral curve (old m, old l, new m and new l)
* appearing as a row in a 2 x 2 matrix:
*
* | <--new m--> | | | | <--old m--> |
* | | = | change_matrices[i] | | |
* | <--new l--> | | | | <--old l--> |
*
* We can invert the change_matrix to solve for the old curves
* in terms of the new ones:
* -1
* | <--old m--> | | | | <--new m--> |
* | | = | change_matrices[i] | | |
* | <--old l--> | | | | <--new l--> |
*
* The Dehn filling curve is
*
* old_m_coef * old_m + old_l_coef * old_l
*
* = old_m_coef * [ change_matrices[i][1][1] * new_m
* - change_matrices[i][0][1] * new_l]
* + old_l_coef * [- change_matrices[i][1][0] * new_m
* + change_matrices[i][0][0] * new_l]
*
* = new_m * [ old_m_coef * change_matrices[i][1][1]
* - old_l_coef * change_matrices[i][1][0] ]
* + new_l * [- old_m_coef * change_matrices[i][0][1]
* + old_l_coef * change_matrices[i][0][0] ]
*
* Therefore
*
* new_m_coef = old_m_coef * change_matrices[i][1][1]
* - old_l_coef * change_matrices[i][1][0]
* new_l_coef = - old_m_coef * change_matrices[i][0][1]
* + old_l_coef * change_matrices[i][0][0]
*/
for (cusp = manifold->cusp_list_begin.next;
cusp != &manifold->cusp_list_end;
cusp = cusp->next)
if (cusp->is_complete == FALSE)
{
old_m_coef = cusp->m;
old_l_coef = cusp->l;
cusp->m = old_m_coef * change_matrices[cusp->index][1][1]
- old_l_coef * change_matrices[cusp->index][1][0];
cusp->l = - old_m_coef * change_matrices[cusp->index][0][1]
+ old_l_coef * change_matrices[cusp->index][0][0];
}
/*
* Update the holonomies according to the rule
*
* | new H(m) | | | | old H(m) |
* | | = | change_matrices[i] | | |
* | new H(l) | | | | old H(l) |
*
* (These are actually logs of holonomies, so it's correct to
* add -- not multiply -- them.)
*
* For complete Cusps the holonomies should be zero, but that's OK.
*/
for (cusp = manifold->cusp_list_begin.next;
cusp != &manifold->cusp_list_end;
cusp = cusp->next)
if ( cusp->topology == torus_cusp || cusp->topology == Klein_cusp ) /* DJH */
for (i = 0; i < 2; i++) /* i = ultimate, penultimate */
{
old_Hm = cusp->holonomy[i][M];
old_Hl = cusp->holonomy[i][L];
cusp->holonomy[i][M] = complex_plus(
complex_real_mult(
change_matrices[cusp->index][0][0],
old_Hm
),
complex_real_mult(
change_matrices[cusp->index][0][1],
old_Hl
)
);
cusp->holonomy[i][L] = complex_plus(
complex_real_mult(
change_matrices[cusp->index][1][0],
old_Hm
),
complex_real_mult(
change_matrices[cusp->index][1][1],
old_Hl
)
);
}
/*
* Update the cusp_shapes.
*/
for (cusp = manifold->cusp_list_begin.next;
cusp != &manifold->cusp_list_end;
cusp = cusp->next)
if ( cusp->topology == torus_cusp || cusp->topology == Klein_cusp ) /* DJH */
{
cusp->cusp_shape[initial] = transformed_cusp_shape(
cusp->cusp_shape[initial],
change_matrices[cusp->index]);
if (cusp->is_complete == TRUE)
cusp->cusp_shape[current] = transformed_cusp_shape(
cusp->cusp_shape[current],
change_matrices[cusp->index]);
/*
* else cusp->cusp_shape[current] == Zero, and needn't be changed.
*/
}
return func_OK;
}
static void current_curve_basis_on_cusp(
Cusp *cusp,
MatrixInt22 basis_change)
{
int m_int,
l_int,
the_gcd;
long a,
b;
Complex new_shape;
int multiple;
int i,
j;
m_int = (int) cusp->m;
l_int = (int) cusp->l;
if (cusp->is_complete == FALSE /* cusp is filled and */
&& m_int == cusp->m /* coefficients are integers */
&& l_int == cusp->l)
{
/*
* Find a and b such that am + bl = gcd(m, l).
*/
the_gcd = euclidean_algorithm(m_int, l_int, &a, &b);
/*
* Divide through by the g.c.d.
*/
m_int /= the_gcd;
l_int /= the_gcd;
/*
* Set basis_change to
*
* m l
* -b a
*/
basis_change[0][0] = m_int;
basis_change[0][1] = l_int;
basis_change[1][0] = -b;
basis_change[1][1] = a;
/*
* Make sure the new longitude is as short as possible.
* The ratio (new longitude)/(new meridian) should have a
* real part in the interval (-1/2, +1/2].
*/
/*
* Compute the new_shape, using the tentative longitude.
*/
new_shape = transformed_cusp_shape( cusp->cusp_shape[initial],
basis_change);
/*
* 96/10/1 There is a danger that for nonhyperbolic solutions
* the cusp shape will be ill-defined (either very large or NaN).
* However for some nonhyperbolic solutions (flat solutions
* for example) it may make good sense. So we attempt to
* make the longitude short iff the new_shape is defined and
* not outrageously large; otherwise we're content with an
* arbitrary longitude.
*/
if (complex_modulus(new_shape) < BIG_MODULUS)
{
/*
* Figure out how many meridians we need to subtract
* from the longitude.
*/
multiple = (int) floor(new_shape.real - (-0.5 + EPSILON));
/*
* longitude -= multiple * meridian
*/
for (j = 0; j < 2; j++)
basis_change[1][j] -= multiple * basis_change[0][j];
}
}
else
{
/*
* Set basis_change to the identity.
*/
for (i = 0; i < 2; i++)
for (j = 0; j < 2; j++)
basis_change[i][j] = (i == j);
}
}
