add fps_is_invertible

Author: Quentin18

examples/example12.c

@@ -16,6 +16,7 @@ int main()
     modulus m = 7;
 
     fps_print(s, n, "S");
+    printf("S invertible? -> %d\n", fps_is_invertible(s));
     fps_newton_inverse(s, n, u, m);
 
     return 0;

lib/fps.c

@@ -64,6 +64,17 @@ void fps_print(const fps s, prec n, char *name)
     }
 }
 
+/**
+ * Returns 1 if the formal power series s is invertible (if s0 != 0), 0 else.
+ * 
+ * @param s fps
+ * @return 1 if the fps is invertible, 0 else
+ */
+int fps_is_invertible(const fps s)
+{
+    return s[0] == 0 ? 0 : 1;
+}
+
 /**
  * Inverts a formal power series using the Newton's method.
  * 

lib/fps.h

@@ -17,4 +17,5 @@ typedef polynomial fps;
 typedef unsigned int prec;
 
 void fps_print(const fps s, prec n, char *name);
+int fps_is_invertible(const fps s);
 void fps_newton_inverse(const fps s, prec n, fps u, modulus m);