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 A051281 #35 Feb 16 2020 01:01:12
%S A051281 1,3,7,31,127,217,889,2667,3937,8191,27559,57337,131071,172011,253921,
%T A051281 524287,917497,1040257,1777447,3670009,4063201,11010027,12189603,
%U A051281 16252897,16646017,66584449,113770279,116522119,225735769,677207307,1073602561,2147483647,3612185689,4294434817,7515217927
%N A051281 Sum of divisors of n, sigma(n) (A000203), is a power of number of divisors of n, d(n) (A000005).
%C A051281 All Mersenne primes (A000668) are terms.
%C A051281 Subsequence of A046528 (product of distinct Mersenne primes). - _Michel Marcus_, Feb 15 2020
%H A051281 David A. Corneth, <a href="/A051281/b051281.txt">Table of n, a(n) for n = 1..2000</a>
%e A051281 d(217) = 4; sigma(217) = 256 = 4^4.
%t A051281 spdQ[n_]:=Module[{sd=DivisorSigma[1,n],nd=DivisorSigma[0,n]},sd == nd^IntegerExponent[sd,nd]]; Join[{1},Select[Range[2,226000000],spdQ]] (* _Harvey P. Dale_, May 02 2012 *)
%o A051281 (PARI) is(n)=my(t,e=ispower(sigma(n),,&t)); if(!e,return(n==1),nd); nd=numdiv(n); fordiv(e,d,if(t^d==nd,return(1)));0 \\ _Charles R Greathouse IV_, Feb 19 2013
%o A051281 (PARI) isA051281(n) = { if(n==1, return(1)); my(sig = sigma(n), ndiv = numdiv(n), v = valuation(sig, ndiv)); (ndiv^v == sig); } \\ _Antti Karttunen_, Jun 30 2017
%Y A051281 Cf. A000005, A000203, A046528, A286628, A289276.
%K A051281 nonn,nice
%O A051281 1,2
%A A051281 _Michel ten Voorde_
%E A051281 More terms from _Jud McCranie_
%E A051281 a(30)-a(32) from _Donovan Johnson_, Oct 03 2012
%E A051281 a(33)-a(35) from _Michel Marcus_, Feb 14 2020