Chinese remaindering also works for m1=1 or m2=1

flintlib · Oct 21, 2023 · aa90012 · aa90012
1 parent 4293d5c
commit aa90012
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Showing 4 changed files with 3 additions and 15 deletions.
diff --git a/doc/source/fmpz.rst b/doc/source/fmpz.rst
@@ -1283,8 +1283,7 @@ The ``fmpz_multi_CRT`` class is similar to ``fmpz_multi_CRT_ui`` except that it
     congruent to `r_1` modulo `m_1` and `r_2` modulo `m_2`,
     where `M = m_1 \times m_2`. The result `x` is stored in ``out``.
 
-    It is assumed that `m_1` and `m_2` are positive integers greater
-    than `1` and coprime.
+    It is assumed that `m_1` and `m_2` are positive coprime integers.
 
     If sign = 0, it is assumed that `0 \le r_1 < m_1` and `0 \le r_2 < m_2`.
     Otherwise, it is assumed that `-m_1 \le r_1 < m_1` and `0 \le r_2 < m_2`.
@@ -1296,8 +1295,7 @@ The ``fmpz_multi_CRT`` class is similar to ``fmpz_multi_CRT_ui`` except that it
     congruent to `r_1` modulo `m_1` and `r_2` modulo `m_2`,
     where `M = m_1 \times m_2`.
 
-    It is assumed that `m_1` and `m_2` are positive integers greater
-    than `1` and coprime.
+    It is assumed that `m_1` and `m_2` are positive coprime integers.
 
     If sign = 0, it is assumed that `0 \le r_1 < m_1` and `0 \le r_2 < m_2`.
     Otherwise, it is assumed that `-m_1 \le r_1 < m_1` and `0 \le r_2 < m_2`.

diff --git a/src/fmpz/CRT.c b/src/fmpz/CRT.c
@@ -72,9 +72,7 @@ void fmpz_CRT(fmpz_t out, const fmpz_t r1, const fmpz_t m1,
     fmpz_init(c);
 
     fmpz_mod(c, m1, m2);
-    fmpz_invmod(c, c, m2);
-
-    if (fmpz_is_zero(c))
+    if (!fmpz_invmod(c, c, m2))
     {
         flint_printf("Exception (fmpz_CRT). m1 not invertible modulo m2.\n");
         flint_abort();

diff --git a/src/fmpz/CRT_ui.c b/src/fmpz/CRT_ui.c
@@ -61,12 +61,6 @@ void fmpz_CRT_ui(fmpz_t out, const fmpz_t r1, const fmpz_t m1,
     c = fmpz_fdiv_ui(m1, m2);
     c = n_invmod(c, m2);
 
-    if (c == 0)
-    {
-        flint_printf("Exception (fmpz_CRT_ui). m1 not invertible modulo m2.\n");
-        flint_abort();
-    }
-
     fmpz_init(m1m2);
     fmpz_mul_ui(m1m2, m1, m2);
 

diff --git a/src/ulong_extras/test/t-CRT.c b/src/ulong_extras/test/t-CRT.c
@@ -37,9 +37,7 @@ int main(void)
         fmpz_randtest_not_zero(m1, state, b1);
         fmpz_randtest_not_zero(m2, state, b2);
         fmpz_abs(m1, m1);
-        fmpz_add_ui(m1, m1, 1);
         fmpz_abs(m2, m2);
-        fmpz_add_ui(m2, m2, 1);
         fmpz_mul(m1m2, m1, m2);
 
         fmpz_gcd(r, m1, m2);