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

Author

Lucas A. Brown, Oct 15 2022

Keywords

• nonn
• hard

Comments

a(30) > 8040423200947.

Examples

			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.
		

Links

• Lucas A. Brown, semiprimemods.py.

Crossrefs

Cf. A001358, A106132.

Formula

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