A357807 Semiprimes k such that k is congruent to 3 modulo k's index in the sequence of semiprimes.

4, 9, 15, 111, 141, 237, 27663, 27667, 3066878, 3066893, 3067023, 3067033, 3067073, 3067193, 3067243, 3067273, 3067283, 3067543, 3067598, 3067613, 3067663, 3067798, 3067843, 3067853, 3067913, 3067933, 3067993, 348933171, 348933219, 348933297

Offset: 1

Author

Lucas A. Brown, Oct 13 2022

Keywords

• nonn
• hard

Comments

a(48) > 8040423200947.

Examples

			The 1st semiprime is 4, which is congruent to 3 (mod 1), so 4 is in the sequence.
The 2nd semiprime is 6, which is not congruent to 3 (mod 2), so 6 is not in the sequence.
The 3rd semiprime is 9, which is congruent to 3 (mod 3), so 9 is in the sequence.
The 4th semiprime is 10, which is not congruent to 3 (mod 4), so 10 is not in the sequence.
		

Links

• Lucas A. Brown, Table of n, a(n) for n = 1..47
• Lucas A. Brown, semiprimemods.py

Crossrefs

Cf. A001358, A106128.

Formula

a(n) = A001358(A106128(n)).