A356357 Semiprimes k such that k is congruent to 7 modulo k's index in the sequence of semiprimes

4, 21, 25, 205, 26707, 27679, 3066877, 3067067, 3067097, 3067117, 3067147, 3067177, 3067557, 3067567, 3067577, 3067607, 3067717, 348933193, 348933421, 348933439, 44690978633, 44690978899, 6553736049327, 6553736049407, 6553736049599, 6553736049631, 6553736049823, 6553736053327, 6553736054959

Offset

1,1

Comments

a(30) > 8040423200947.

Links

Table of n, a(n) for n=1..29.

Lucas A. Brown, semiprimemods.py.

Formula

a(n) = A001358(A106132(n)).

Example

The 1st semiprime is 4, which is congruent to 7 (mod 1), so 4 is in the sequence.
The 2nd semiprime is 6, which is not congruent to 7 (mod 2), so 6 is not in the sequence.
The 3rd semiprime is 9, which is not congruent to 7 (mod 3), so 9 is not in the sequence.
The 7th semiprime is 21, which is congruent to 7 (mod 7), so 21 is in the sequence.

Crossrefs

Cf. A001358, A106132.

Sequence in context: A133784 A041821 A042429 * A276400 A304770 A316513

Adjacent sequences: A356354 A356355 A356356 * A356358 A356359 A356360

Keyword

nonn,hard

Author

Lucas A. Brown, Oct 15 2022

Status

approved