cp's OEIS Frontend

A249397 Composite numbers whose Euler totient divides the sum of the Euler totients of the numbers less than or equal to n and not relatively prime to n.

%I A249397 #24 Nov 11 2014 21:05:11
%S A249397 161,171,895,1337,1843,1967,2575,5833,8255,36121,54439,87353,195921,
%T A249397 274115,284419,340363,368449,387087,444639,504539
%N A249397 Composite numbers whose Euler totient divides the sum of the Euler totients of the numbers less than or equal to n and not relatively prime to n.
%C A249397 No more terms < 10^6.
%e A249397 Numbers not coprime to 161 are 7, 14, 21, 23, 28, 35, 42, 46, 49, 56, 63, 69, 70, 77, 84, 91, 92, 98, 105, 112, 115, 119, 126, 133, 138, 140, 147, 154, 161 and the sum of their Euler totients is 1320; phi(161) = 132 and 1320/132 = 10.
%p A249397 with(numtheory): P:=proc(q) local a,k,n; for n from 1 to q do
%p A249397 if not isprime(n) then a:=0;
%p A249397 for k from 1 to n do if gcd(k,n)>1 then a:=a+phi(k); fi; od;
%p A249397 if type(a/phi(n),integer) then print(n); fi; fi; od; end: P(10^9);
%o A249397 (PARI) isok(n) = (n!=1) && !isprime(n) && (sum(k=1, n-1, if (gcd(k, n) != 1, eulerphi(k), 0)) % eulerphi(n) == 0); \\ _Michel Marcus_, Oct 29 2014
%Y A249397 Cf. A000010, A249396.
%K A249397 nonn,more
%O A249397 1,1
%A A249397 _Paolo P. Lava_, Oct 27 2014
%E A249397 a(11)-a(12) from _Michel Marcus_, Nov 01 2014
%E A249397 a(13)-a(19) from _Michel Marcus_, Nov 03 2014
%E A249397 a(20) from _Ray Chandler_, Nov 04 2014