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 A115559 #16 Jan 28 2025 01:47:08 %S A115559 1,316,333,549,844,963,981,1052,1233,1251,1304,1341,1359,1474,1629, %T A115559 1688,1737,1738,1996,2061,2144,2216,2421,2528,2547,2763,2979,3033, %U A115559 3082,3123,3141,3148,3231,3244,3283,3303,3411,3573,3634,3871,3879,3897,3988,4113 %N A115559 Nonprime terms of A115558. %H A115559 Giovanni Resta, <a href="/A115559/b115559.txt">Table of n, a(n) for n = 1..10000</a> %F A115559 The commutator [sigma, tau] is zero and a(n) is the square root of solutions. Moreover this square root is a nonprime number. %t A115559 Select[Range[5000], !PrimeQ[#] && DivisorSigma[0, DivisorSigma[1, #^2]] == DivisorSigma[1, DivisorSigma[0, #^2]] &] (* _Amiram Eldar_, Jan 28 2025 *) %o A115559 (PARI) isok(n)= !isprime(n) && (sigma(numdiv(n^2)) == numdiv(sigma(n^2))); \\ _Michel Marcus_, Dec 20 2013 %Y A115559 Intersection of A018252 and A115558. %Y A115559 Cf. A000005, A000203, A076360, A076361, A062068, A062069, A115557, A115560. %K A115559 nonn %O A115559 1,2 %A A115559 _Labos Elemer_, Jan 25 2006