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 A015840 #26 Feb 12 2021 01:30:28 %S A015840 1,2,3,4,7,8,12,14,19,20,21,24,27,30,32,39,52,63,84,91,102,112,117, %T A015840 127,129,132,135,155,165,181,200,210,252,327,348,387,403,405,410,432, %U A015840 452,453,456,459,475,480,532,549,592,620,624,679,693,714,725,952,1009 %N A015840 Numbers k such that phi(k) | sigma(k + 3). %H A015840 Ivan Neretin, <a href="/A015840/b015840.txt">Table of n, a(n) for n = 1..1000</a> %p A015840 with(numtheory): A015840:=n->`if`(sigma(n+3) mod phi(n) = 0, n, NULL): seq(A015840(n), n=1..5*10^3); # _Wesley Ivan Hurt_, Feb 04 2017 %t A015840 Select[Range[3000], Divisible[DivisorSigma[1, 3 + #], EulerPhi[#]] &] (* _David Nacin_, Mar 03 2012 *) %o A015840 (PARI) is(n)=!(sigma(n+3)%eulerphi(n)) \\ _Charles R Greathouse IV_, Sep 25 2012 %Y A015840 Cf. A020492. %K A015840 nonn %O A015840 1,2 %A A015840 _Robert G. Wilson v_ %E A015840 a(1) = 1 inserted by _Ivan Neretin_, Jan 30 2017