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fix the bound for the coeffs of minpoly #2480

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fix the bound for the coeffs of minpoly #2480

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817086b
fix the bound for the coeffs of minpoly
fieker Nov 7, 2025
55aeedc
remove debugging
fieker Nov 7, 2025
c660e2a
change the parameters to exercise the error
fieker Nov 10, 2025
47b7901
try a correct product...
fieker Nov 13, 2025
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21 changes: 18 additions & 3 deletions src/fmpz_mat/minpoly_modular.c
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Original file line number Diff line number Diff line change
Expand Up @@ -42,7 +42,7 @@ static slong _fmpz_mat_minpoly_small(fmpz * rop, const fmpz_mat_t op)
static void _fmpz_mat_bound_ovals_of_cassini(fmpz_t b, const fmpz_mat_t op)
{
slong n = op->r, i, j;
fmpz * v1;
fmpz * v1, * v2;
fmpz_t t, q, r1, r2;

fmpz_init(t);
Expand All @@ -51,6 +51,7 @@ static void _fmpz_mat_bound_ovals_of_cassini(fmpz_t b, const fmpz_mat_t op)
fmpz_init(r2);

v1 = _fmpz_vec_init(n);
v2 = _fmpz_vec_init(n);

/* |A| [1,1,...,1]^T */
for (i = 0; i < n; i++)
Expand All @@ -63,13 +64,27 @@ static void _fmpz_mat_bound_ovals_of_cassini(fmpz_t b, const fmpz_mat_t op)
fmpz_sub(v1 + i, v1 + i, fmpz_mat_entry(op, i, j));
}
}
/* |A|^t * |A| * [1,1,...,1]^T */
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@fredrik-johansson fredrik-johansson Nov 13, 2025

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The code doesn't seem to match the formula in the comment. It looks like v1[i] is being repeatedly multiplied by different matrix entries while the intended behavior is to repeatedly add products of different matrix entries.

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@fieker fieker Nov 13, 2025

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good catch...I've tried again, with a better matrix vector product

for (i = 0; i < n; i++)
{
for (j = 0; j < n; j++)
{
if (fmpz_sgn(fmpz_mat_entry(op, j, i)) >= 0)
fmpz_addmul(v2 + i, v1 + j, fmpz_mat_entry(op, j, i));
else
fmpz_submul(v2 + i, v1 + j, fmpz_mat_entry(op, j, i));
}
}

_fmpz_vec_clear(v1, n);
v1 = v2;

for (i = 0; i < n; i++)
{
fmpz_zero(t);

/* q_i */
fmpz_abs(t, fmpz_mat_entry(op, i, i));
for (j = 1; j < n; j++)
fmpz_addmul(t, fmpz_mat_entry(op, i, j), fmpz_mat_entry(op, i, j));

if (fmpz_cmp(t, q) > 0)
fmpz_set(q, t);
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2 changes: 1 addition & 1 deletion src/fmpz_mat/test/t-minpoly.c
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Expand Up @@ -52,7 +52,7 @@ TEST_FUNCTION_START(fmpz_mat_minpoly, state)
fmpz_poly_init(r);

if (n_randint(state, 50) == 0)
fmpz_mat_randtest(A, state, 1000);
fmpz_mat_randtest(A, state, 10000);
else
fmpz_mat_randtest(A, state, 10);

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