This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A360697 #31 Mar 02 2023 22:59:52 %S A360697 0,1,4,9,4,4,4,1,4,4,1,2,5,1,4,4,4,4,4,1,4,5,8,1,4,4,4,4,1,4,9,1,1,4, %T A360697 4,4,4,4,4,4,4,4,4,4,1,4,4,4,4,1,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4, %U A360697 1,4,1,4,4,4,4,4,4,4,2,1,4,4,1,4,4,4,1,2,4,4,4,1,4,4,1,4,4,1,4,4,1 %N A360697 The sum of the squares of the digits of n, repeated until reaching a single-digit number. %C A360697 Square the digits of n, then sum the squares. Repeat the process until the sum is less than 10. %e A360697 For n=28, the sum of the squares of the digits gives 4+64 = 68. Repeating the process gives 36+64 = 100; repeating once more gives 1+0+0 = 1. Therefore a(28) is 1. %e A360697 a(n) = 4 for 72 of the first 100 n (0 to 99 inclusive.) %t A360697 f[n_] := Plus @@ (IntegerDigits[n]^2); a[n_] := NestWhile[f, f[n], # > 9 &]; Array[a, 100, 0] (* _Amiram Eldar_, Feb 17 2023 *) %Y A360697 Cf. A003132, A007770, A099645. %K A360697 nonn,base %O A360697 0,3 %A A360697 _Will Nicholes_, Feb 16 2023