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 A188540 #17 May 23 2023 15:08:40 %S A188540 1,13,19,22,31,37,38,53,67,83,86,89,109,113,124,127,131,133,134,139, %T A188540 148,157,169,181,187,199,211,233,251,253,257,263,292,293,295,307,310, %U A188540 317,326,328,337,338,343,353,355,361,376,379,389,401,406,409,412,422,427,438,443,449,453 %N A188540 Numbers k such that d(k+2) = 2*d(k) where d(k) is the number of divisors of k (A000005). %H A188540 Amiram Eldar, <a href="/A188540/b188540.txt">Table of n, a(n) for n = 1..10000</a> %e A188540 1 is a term since d(1+2) = d(3) = 2 = 2*d(1). %t A188540 Select[Range[500], DivisorSigma[0, # + 2] == 2 * DivisorSigma[0, #] &] (* _Amiram Eldar_, Jan 17 2021 *) %t A188540 Position[Partition[DivisorSigma[0,Range[500]],3,1],_?(2#[[1]]== #[[3]]&),1,Heads-> False]// Flatten (* _Harvey P. Dale_, Oct 19 2021 *) %o A188540 (PARI) isok(k) = numdiv(k+2) == 2*numdiv(k); \\ _Michel Marcus_, Jan 17 2021 %Y A188540 Cf. A000005, A063446, A160921. %K A188540 nonn,easy %O A188540 1,2 %A A188540 _Juri-Stepan Gerasimov_, Apr 03 2011