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 A189941 #9 Mar 31 2012 10:28:21 %S A189941 186,258,322,338,3866,4326,4775,18830,19122,27586,34330,34538,41626, %T A189941 46762,49858,49922,54298,55810,70510,82122,86938,89102,101042,101706, %U A189941 106442,110510,112910,118586,120822,129722,133430,134714,150742,157362,158235,163410 %N A189941 Numbers n such that n''' = n''+ 1 where n'' and n''' are respectively the second and the third arithmetic derivative of n. %C A189941 The second arithmetic derivative of a(n) is a Giuga's number A007850 (solution of n'=n+1). %H A189941 Donovan Johnson, <a href="/A189941/b189941.txt">Table of n, a(n) for n = 1..673</a> %e A189941 186'= 161; 186"=161' = 30; 186"'=30'= 31-> 186'''= 186" +1 -> a(1)=186. %p A189941 Using Porter's code from A003415 der:=n->n*add(op(2,p)/op(1,p),p=ifactors(n)[2]) %p A189941 for i from 1 to n do a:=der(der(der(i)))-der(der(i))-1: if a=0 then j:=j+1; A[j]:=i: end if od %Y A189941 Cf. A003415, A007850. %K A189941 nonn %O A189941 1,1 %A A189941 _Giorgio Balzarotti_, May 01 2011