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Odd terms in A015782.
0

%I #11 Jan 24 2022 16:04:44

%S 7,21,93,357,381,681,1541,7181,24573,36893,192061,388669,393213,

%T 1505533,1572861,10678781,24736253

%N Odd terms in A015782.

%C Odd numbers n such that phi(n)+3 divides sigma(n+3), where phi = A000010 and sigma = A000203.

%e a(3) = 93 is a term because phi(93)+3 = 63 divides sigma(96) = 252 = 4*63.

%p filter:= proc(n) uses numtheory;

%p sigma(n+3) mod (3+phi(n)) = 0

%p end proc:

%p select(filter, [seq(i,i=1..2*10^6,2)]);

%t Select[Range[1, 2*10^6, 2], Divisible[DivisorSigma[1, # + 3], EulerPhi[#] + 3] &] (* _Amiram Eldar_, Jan 24 2022 *)

%Y Cf. A000010, A000203, A015782.

%K nonn,more

%O 1,1

%A _Robert Israel_, Jan 24 2022

%E a(16)-a(17) from _Amiram Eldar_, Jan 24 2022