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 A053248 #9 Oct 27 2023 22:00:45 %S A053248 1,4,10,9,11,30,10,20,40,32,16,65,13,42,82,46,12,96,26,82,106,46,18, %T A053248 143,57,70,114,88,40,214,26,66,136,88,120,244,30,60,160,171,47,256,36, %U A053248 150,266,96,40,286,88,176,226,124,12,24,342,170,230,222,94,34,443,71 %N A053248 Second differences of sigma(n) that are negative. %F A053248 a(n) = A053223(A053229(n)). %p A053248 with (numtheory): f := [seq( sigma(i+2) - sigma(i+1) - (sigma(i+1) - sigma(i)), i=1..200)]; %p A053248 seq(`if`(f[i] < 0,f[i],print( )), i=1..200); %t A053248 Abs[Select[Differences[DivisorSigma[1,Range[200]],2],Negative]] (* _Harvey P. Dale_, Aug 13 2015 *) %Y A053248 Cf. A000203, A053223, A053247. %K A053248 nonn %O A053248 1,2 %A A053248 _Asher Auel_, Jan 11 2000