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 A261672 #39 Jun 28 2023 09:20:10 %S A261672 4,7,52,100,136,388,30940,33250 %N A261672 Numbers k such that A037610(k) is prime. %C A261672 The terms are a subset of the terms of A016777, since a term of A037610 can only be prime if it is congruent to 1 modulo 10 and hence congruent to 1 modulo 3. If A037610(k) is congruent to 1 modulo 3, then k is congruent to 1 modulo 3 as well. %C A261672 No further terms up to 10000. %e A261672 A037610(7) = 1231231 is prime, so 7 is a term of the sequence. %t A261672 Select[Range@ 500, PrimeQ@ Floor[41/333*10^#] &] (* _Michael De Vlieger_, Sep 07 2015 *) %o A261672 (PARI) a037610(n) = 10^n*41\333 %o A261672 is(n) = ispseudoprime(a037610(n)) %Y A261672 Cf. A037610, A062209. %K A261672 nonn,more %O A261672 1,1 %A A261672 _Felix Fröhlich_, Sep 04 2015 %E A261672 a(7)-a(8) from _Michael S. Branicky_, Jun 28 2023