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%I #17 Aug 27 2015 08:17:12
%S 2,777,98433,214388517,152739317354
%N Numbers n such that n'=(n+1)', where n' is the arithmetic derivative of n.
%C a(6) > 10^13. - _Giovanni Resta_, Mar 13 2014
%C a(6) > 5*10^13. - _Hiroaki Yamanouchi_, Aug 27 2015
%e 2' = 3' = 1
%e 777' = 778' = 391
%e 98433' = 98434' = 65631
%e 214388517' = 214388518' = 107194261
%p with(numtheory); ListA226779:=proc(q) local a,b,n,p; a:=0;
%p for n from 2 to q do b:=n*add(op(2,p)/op(1,p),p=ifactors(n)[2]);
%p if a=b then print(n-1); fi; a:=b; od; end: ListA226779(10^12);
%Y Cf. A003415
%K nonn,more
%O 1,1
%A _Paolo P. Lava_, Jun 18 2013
%E a(5) from _Giovanni Resta_, Jun 18 2013