OFFSET
1,1
COMMENTS
The second arithmetic derivative of a(n) is a Giuga's number A007850 (solution of n'=n+1).
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..673
EXAMPLE
186'= 161; 186"=161' = 30; 186"'=30'= 31-> 186'''= 186" +1 -> a(1)=186.
MAPLE
Using Porter's code from A003415 der:=n->n*add(op(2, p)/op(1, p), p=ifactors(n)[2])
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
CROSSREFS
KEYWORD
nonn
AUTHOR
Giorgio Balzarotti, May 01 2011
STATUS
approved