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 A074821 #19 May 08 2025 02:23:53 %S A074821 2,3,4,5,10,11,23,27,29,34,38,41,46,53,55,57,62,76,77,83,89,91,93,106, %T A074821 113,118,123,129,131,133,136,143,145,159,161,173,177,179,185,191,201, %U A074821 203,205,206,212,213,218,226,232,233,235,239,251,259,267,281,291,293 %N A074821 Numbers k such that tau(k) = tau(2k+1). %C A074821 If a term is a perfect square, then its square root must be in A001542. The first few such terms are the squares of 2, 2378, 93222358, 165326326037771920630... - _Ivan Neretin_, May 25 2016 %H A074821 Ivan Neretin, <a href="/A074821/b074821.txt">Table of n, a(n) for n = 1..10014</a> %t A074821 Select[Range[300], DivisorSigma[0, #] == DivisorSigma[0, 2 # + 1] &] (* _Ivan Neretin_, May 25 2016 *) %o A074821 (PARI) isok(k) = numdiv(k) == numdiv(2*k+1); \\ _Amiram Eldar_, May 08 2025 %Y A074821 Cf. A000005 (tau), A001542, A005384 (subsequence), A099774. %K A074821 nonn %O A074821 1,1 %A A074821 _Benoit Cloitre_, Sep 08 2002