This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A356357 #8 Jul 23 2025 16:03:57 %S A356357 4,21,25,205,26707,27679,3066877,3067067,3067097,3067117,3067147, %T A356357 3067177,3067557,3067567,3067577,3067607,3067717,348933193,348933421, %U A356357 348933439,44690978633,44690978899,6553736049327,6553736049407,6553736049599,6553736049631,6553736049823,6553736053327,6553736054959 %N A356357 Semiprimes k such that k is congruent to 7 modulo k's index in the sequence of semiprimes. %C A356357 a(30) > 8040423200947. %H A356357 Lucas A. Brown, <a href="https://github.com/lucasaugustus/oeis/blob/main/semiprimemods.py">semiprimemods.py</a>. %F A356357 a(n) = A001358(A106132(n)). %e A356357 The 1st semiprime is 4, which is congruent to 7 (mod 1), so 4 is in the sequence. %e A356357 The 2nd semiprime is 6, which is not congruent to 7 (mod 2), so 6 is not in the sequence. %e A356357 The 3rd semiprime is 9, which is not congruent to 7 (mod 3), so 9 is not in the sequence. %e A356357 The 7th semiprime is 21, which is congruent to 7 (mod 7), so 21 is in the sequence. %Y A356357 Cf. A001358, A106132. %K A356357 nonn,hard %O A356357 1,1 %A A356357 _Lucas A. Brown_, Oct 15 2022