sq12: extend to minimum requirement for chain length 11

Except for some special cases handled separately, n cannot have 12 divisors
if it is in {8,16,24} (mod 32), so chains of length 10 must match values
[-2, -1, 0, 1, 2] (mod 32). But we're looking for chains of length 11 now,
so we can add [-3, 3] (mod 32) to the list of requirements, and reduce
the number of cases that need checking manually.

Author: hvds

sq12.c

@@ -181,6 +181,13 @@ void tryvalue(mpz_t zv) {
     if (!is_taux(Z(temp), 12, 1))
         return;
     mpz_add_ui(Z(temp), Z(n32), 1);
+    if (!is_taux(Z(temp), 12, 1))
+        return;
+    /* extend chain 10 to 11 */
+    mpz_sub_ui(Z(temp), Z(n32), 3);
+    if (!is_taux(Z(temp), 12, 1))
+        return;
+    mpz_add_ui(Z(temp), Z(n32), 3);
     if (!is_taux(Z(temp), 12, 1))
         return;
     keep_diag();