Add square_pyramidal_numbers (#524)

Author: jojohanhannesnes

src/math/mod.rs

@@ -39,6 +39,7 @@ pub mod sieve_of_eratosthenes;
 mod signum;
 mod simpson_integration;
 mod sine;
+mod square_pyramidal_numbers;
 mod square_root;
 mod sum_of_digits;
 mod trial_division;
@@ -91,6 +92,7 @@ pub use self::sieve_of_eratosthenes::sieve_of_eratosthenes;
 pub use self::signum::signum;
 pub use self::simpson_integration::simpson_integration;
 pub use self::sine::sine;
+pub use self::square_pyramidal_numbers::square_pyramidal_number;
 pub use self::square_root::{fast_inv_sqrt, square_root};
 pub use self::sum_of_digits::{sum_digits_iterative, sum_digits_recursive};
 pub use self::trial_division::trial_division;

src/math/square_pyramidal_numbers.rs

@@ -0,0 +1,20 @@
+// https://en.wikipedia.org/wiki/Square_pyramidal_number
+// 1² + 2² + ... = ... (total)
+
+pub fn square_pyramidal_number(n: u64) -> u64 {
+    n * (n + 1) * (2 * n + 1) / 6
+}
+
+#[cfg(test)]
+mod tests {
+
+    use super::*;
+
+    #[test]
+    fn test0() {
+        assert_eq!(0, square_pyramidal_number(0));
+        assert_eq!(1, square_pyramidal_number(1));
+        assert_eq!(5, square_pyramidal_number(2));
+        assert_eq!(14, square_pyramidal_number(3));
+    }
+}