A357023 - OEIS

%I #32 Oct 16 2022 03:23:41

%S 4,185,206,209,27681,3066905,3067135,3067795,3067985,348933197,

%T 348933239,348933251,348933257,348933269,44690978141,44690978162,

%U 44690978519,44690978561,44690978617,44690978869,44690978981,44690979457,44690979527,6553736049293

%N Semiprimes k such that k is congruent to 5 modulo k's index in the sequence of semiprimes.

%C a(32) > 8040423200947.

%H Lucas A. Brown, <a href="/A357023/b357023.txt">Table of n, a(n) for n = 1..31</a>

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

%F a(n) = A001358(A106130(n)).

%e The 1st semiprime is 4, which is congruent to 5 (mod 1), so 4 is in the sequence.

%e The 2nd semiprime is 6, which is not congruent to 5 (mod 6), so 6 is not in the sequence.

%e The 60th semiprime is 185, which is congruent to 5 (mod 60), so 185 is in the sequence.

%Y Cf. A001358, A106130.

%K nonn,hard

%O 1,1

%A _Lucas A. Brown_, Oct 14 2022