**661**is a prime number.

**661**has a representation as a sum of two squares:

**661**= 6

^{2}+ 25

^{2}.

659 and

**661**form a twin prime pair.**661**is the hypotenuse of a primitive Pythagorean triple:

**661**

^{2}= 300

^{2}+ 589

^{2}.

**661**is a centered decagonal number.

Source: Prime Curios!