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 A072858 #24 Nov 18 2023 22:24:15 %S A072858 2,3,5,17,101,163,257,577,883,1297,1801,3137,3529,5477,7057,7351,8929, %T A072858 9397,11831,12101,13457,13553,14401,15361,15377,15973,19841,20809, %U A072858 21401,21601,23549,24337,25601,29401,30977,33301,33751,33857,38237 %N A072858 Primes p such that the period of the decimal expansion of 1/p is a square. %H A072858 Amiram Eldar, <a href="/A072858/b072858.txt">Table of n, a(n) for n = 1..10000</a> %e A072858 The period of 1/17 = 0.05882352941176470588... is 16 = 4^2, hence 17 is in the sequence. %e A072858 The period of 1/163 = 81 = 9^2. %t A072858 Select[Prime[Range[4000]], IntegerQ @ Sqrt[Length[RealDigits[1/#][[1, 1]]]] &] (* _Amiram Eldar_, May 21 2022 *) %o A072858 (PARI) (a(n)=if(n<4,n==2,znorder(Mod(10, prime(n))))); for(n=1,1000,if(issquare(a(n))==1,print1(prime(n),","))) /* Thanks to _Michael Somos_ for improvement of the PARI program */ %Y A072858 Cf. A002371. %K A072858 easy,nonn,base %O A072858 1,1 %A A072858 _Benoit Cloitre_, Jul 26 2002