Centered decagonal number

A centered decagonal number is a centered figurate number that represents a decagon with a dot in the center and all other dots surrounding the center dot in successive decagonal layers. The centered decagonal number for n is given by the formula 5 n 2 − 5 n + 1 {\displaystyle 5n^{2}-5n+1\,} Thus, the first few centered decagonal numbers are 1, 11, 31, 61, 101, 151, 211, 281, 361, 451, 551, 661, 781, 911, 1051, ...

Source: Wikipedia — Centered decagonal number (CC BY-SA 4.0)