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 A015819 #30 Dec 13 2018 12:48:33 %S A015819 3,6,7,11,15,19,21,23,27,31,33,42,43,51,57,59,62,69,70,71,77,79,83,84, %T A015819 87,93,103,131,139,141,159,163,165,177,187,189,191,199,211,213,223, %U A015819 235,237,251,267,270,271,282,285,287,297,311,315,316,321,330,331 %N A015819 Numbers k such that phi(k + 3) | sigma(k). %H A015819 Vincenzo Librandi, <a href="/A015819/b015819.txt">Table of n, a(n) for n = 1..1000</a> %p A015819 select(t -> (numtheory:-sigma(t)/numtheory:-phi(t+3))::integer, [$1..1000]); # _Robert Israel_, Dec 13 2018 %t A015819 Select[Range[1000], Divisible[DivisorSigma[1, #], EulerPhi[3 + #]] &] (* _David Nacin_, Mar 01 2012 *) %o A015819 (PARI) is(n)=sigma(n)%(eulerphi(n+3))==0 \\ _Charles R Greathouse IV_, Sep 25 2012 %Y A015819 Cf. A056097. %Y A015819 Contains A092109. %K A015819 nonn %O A015819 1,1 %A A015819 _Robert G. Wilson v_