cp's OEIS Frontend

A238225 Composite numbers n such that if x = sigma(n)-phi(n)-n then n = sigma(x)-phi(x)-x.

%I A238225 #17 Mar 06 2014 06:31:56
%S A238225 12,42,1242,2010,2358,57990,65802,1782198,1890738,2504920,2733880,
%T A238225 3286128,4181968,6137440,7861200,8233134,11208018,15228688,21445340,
%U A238225 24563812,33714690,38928126,185154858,207494658,221604534,247017798,341398422,366777018
%N A238225 Composite numbers n such that if x = sigma(n)-phi(n)-n then n = sigma(x)-phi(x)-x.
%C A238225 A088826 is a subset of this sequence: it lists the fixed points of the transform n -> sigma(n)-phi(n)-n.
%C A238225 a(29) > 10^9. - _Michel Marcus_, Mar 06 2014
%e A238225 Fixed points: 12, 42, 1242, 6137440, 1385119360, ...
%e A238225 sigma(2010) = 4896, phi(2010) = 528 and 4896 - 528 - 2010 = 2358.
%e A238225 sigma(2358) = 5148, phi(2358) = 780 and 5148 - 780 - 2358 = 2010.
%p A238225 with(numtheory); P:=proc(q)local a,n;
%p A238225 for n from 1 to q do a:=sigma(n)-phi(n)-n;
%p A238225 if a>0 and sigma(a)-phi(a)-a=n then print(n);
%p A238225 fi; od; end: P(10^6);
%o A238225 (PARI) isok(n) = !isprime(n) && ((x = (sigma(n) - eulerphi(n) - n)) > 0) && (n == (sigma(x) - eulerphi(x) - x)); \\ _Michel Marcus_, Mar 06 2014
%Y A238225 Cf. A000010, A000203, A088826, A238226-A238230.
%K A238225 nonn
%O A238225 1,1
%A A238225 _Paolo P. Lava_, Feb 20 2014
%E A238225 a(8)-a(28) from _Michel Marcus_, Mar 06 2014