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 A356357 Semiprimes k such that k is congruent to 7 modulo k's index in the sequence of semiprimes 1
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(30) > 8040423200947. LINKS 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 * A356359 A356360 A356361 KEYWORD nonn,hard AUTHOR Lucas A. Brown, Oct 15 2022 STATUS approved

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Last modified January 29 00:02 EST 2023. Contains 359905 sequences. (Running on oeis4.)