- Added the Number solve_lcg(n, r, m) method.

Returns the smallest solution for `x` to the following linear congruence:

    n*x == r (mod m)

Example:

    say solve_lcg(143, 44, 231)     #=> 10

lib/Sidef/Types/Number/Number.pm

@@ -6344,6 +6344,45 @@ package Sidef::Types::Number::Number {
         return (undef, undef);
     }
 
+    sub solve_lcg {
+        my ($n, $r, $m) = @_;
+
+        # Solve: n*x == r (mod m)
+
+        _valid(\$r, \$m);
+
+        $n = _any2mpz($$n) // goto &nan;
+        $r = _any2mpz($$r) // goto &nan;
+        $m = _any2mpz($$m) // goto &nan;
+
+        Math::GMPz::Rmpz_sgn($m) || goto &nan;
+
+        my $g = Math::GMPz::Rmpz_init();
+        Math::GMPz::Rmpz_gcd($g, $n, $m);
+
+        if (Math::GMPz::Rmpz_cmp_ui($g, 1) != 0) {
+
+            # No solution exists if `r` is NOT divisible by `gcd(n,m)`
+            Math::GMPz::Rmpz_divisible_p($r, $g) || goto &nan;
+
+            $n = Math::GMPz::Rmpz_init_set($n);
+            $r = Math::GMPz::Rmpz_init_set($r);
+            $m = Math::GMPz::Rmpz_init_set($m);
+
+            Math::GMPz::Rmpz_divexact($n, $n, $g);
+            Math::GMPz::Rmpz_divexact($r, $r, $g);
+            Math::GMPz::Rmpz_divexact($m, $m, $g);
+        }
+
+        Math::GMPz::Rmpz_invert($g, $n, $m);
+        Math::GMPz::Rmpz_mul($g, $g, $r);
+        Math::GMPz::Rmpz_mod($g, $g, $m);
+
+        bless \$g;
+    }
+
+    *solve_linear_congruence = \&solve_lcg;
+
     sub sqrt_cfrac_period_each {
         my ($n, $block, $max) = @_;
 

lib/Sidef/Types/Number/Number.pod

@@ -2466,7 +2466,7 @@ For negative values of C<k>, falling factorial is defined as:
 
     falling_factorial(n, -k) = 1/falling_factorial(n + k, k)
 
-When the denominator is zero, NaN is returned.
+When the denominator is zero, C<NaN> is returned.
 
 =cut
 
@@ -5938,7 +5938,7 @@ Example:
     say rat_approx(3.14).as_frac        #=> 22/7
     say rat_approx(zeta(-5)).as_frac    #=> -1/252
 
-Returns NaN when C<x> is not a real number.
+Returns C<NaN> when C<x> is not a real number.
 
 =cut
 
@@ -6225,6 +6225,24 @@ Example:
 
 =cut
 
+=head2 solve_lcg
+
+    solve_lcg(n, r, m)
+
+Return the smallest solution for C<x> to the following linear congruence:
+
+    n*x == r (mod m)
+
+Example:
+
+    say solve_lcg(143, 44, 231)     #=> 10
+
+Return C<NaN> when no solution exists (i.e.: when C<r> is not divisbile by C<gcd(n, m)>).
+
+Aliases: I<solve_linear_congruence>
+
+=cut
+
 =head2 solve_pell
 
     solve_pell(d, k=1)