210125

Appears in sequences

• Numbers k such that k | 4^k + 1.at n=21A015950
• Numbers k that divide 4^k + 1, k not a power of 5.at n=13A015974
• Numbers n that are the hypotenuse of exactly 17 distinct integer-sided right triangles, i.e., n^2 can be written as a sum of two squares in 17 ways.at n=18A097239
• Numbers n which are the hypotenuse of a Pythagorean triple with n' as a leg, where n' is the arithmetic derivative of n.at n=4A211176