A357808 - OEIS

%I #10 Oct 29 2022 04:42:40

%S 4,6,14,115,118,178,187,214,235,3066899,3067069,3067079,3067149,

%T 3067429,3067549,3067594,3067609,3067669,3067719,3067999,44690978147,

%U 44690978217,44690978245,44690978623,44690978903,44690979022,44690979442

%N Semiprimes k such that k is congruent to 4 modulo k's index in the sequence of semiprimes.

%C a(28) > 8040423200947.

%C a(28) <= 1095927464608618, a(29) <= 1095927464608951 and a(38) <= 1095927464630173. - _Martin Ehrenstein_, Oct 28 2022

%H Lucas A. Brown, <a href="https://github.com/lucasaugustus/oeis/blob/main/semiprimemods.py">semiprimemods.py</a>.

%F a(n) = A001358(A106129(n)).

%e The 1st semiprime is 4, which is congruent to 4 (modulo 1), so 4 is in the sequence.

%e The 2nd semiprime is 6, which is congruent to 4 (modulo 2), so 6 is in the sequence.

%e The 3rd semiprime is 9, which is not congruent to 4 (modulo 3), so 9 is not in the sequence.

%e The 4th semiprime is 10, which is not congruent to 4 (modulo 4), so 10 is not in the sequence.

%e The 5th semiprime is 14, which is congruent to 4 (modulo 5), so 14 is in the sequence.

%Y Cf. A001358, A106129.

%K nonn,hard,more

%O 1,1

%A _Lucas A. Brown_, Oct 13 2022