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 A348284 #23 Oct 16 2021 19:27:19 %S A348284 1,2,3,4,5,7,8,11,13,16,17,19,23,24,27,29,31,37,41,43,47,48,53,54,59, %T A348284 61,64,67,71,73,79,83,89,97,101,103,107,108,109,113,127,128,131,137, %U A348284 139,149,151,157,162,163,167,168,173,179,181,191,193,197,199,211,223,227,229,233 %N A348284 Numbers k such that k | k" where k" is the 2nd arithmetic derivative of k. %C A348284 As 1" = 0 and p" = 0 when p is prime, 1 and every prime are terms, hence A008578 is a subsequence. - _Bernard Schott_, Oct 12 2021 %e A348284 8 is in the sequence since 8" = 16 and 8 | 16. %p A348284 d:= n-> n*add(i[2]/i[1], i=ifactors(n)[2]): %p A348284 q:= n-> is(irem(d(d(n)), n)=0): %p A348284 select(q, [$1..250])[]; # _Alois P. Heinz_, Oct 15 2021 %o A348284 (PARI) ad(n) = vecsum([n/f[1]*f[2]|f<-factor(n+!n)~]); \\ A003415 %o A348284 isok(k) = !(ad(ad(k)) % k); \\ _Michel Marcus_, Oct 10 2021 %Y A348284 Cf. A003415 (1st derivative), A068346 (2nd derivative). %Y A348284 Cf. A008578, A072873. %K A348284 nonn %O A348284 1,2 %A A348284 _Wesley Ivan Hurt_, Oct 09 2021