661 is a prime number.
661 has a representation as a sum of two squares: 661 = 62 + 252.
659 and 661 form a twin prime pair.
661 is the hypotenuse of a primitive Pythagorean triple: 6612 = 3002 + 5892.
661 is a centered decagonal number.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg-V0l_uOh2GjDlVguTaYlWbopP-baOFnCMOGz7Xqr6GIBgBsrVewJEXqMrEakSzB7KrFJ0e0lSUetzKazj2J3l3Uww67jiAGDvNNCrZdvXgKgB9UiBU_cND_PFAu3UpsozEbGyKhf9onw/s200/n0661.jpg)
Source: Prime Curios!