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 A363491 #11 Apr 22 2025 16:05:19 %S A363491 7,13,14,16,19,28,30,31,40,42,51,54,55,58,62,68,85,88,96,111,112,116, %T A363491 128,148,160,162,188,192,198,220,222,236,242,276,300,318,319,320,332, %U A363491 372,373,398,420,428,432,458,460,482,505,520,532,542,546,556,650,692,714 %N A363491 Numbers k such that 2^k - 5 is a semiprime. %C A363491 The numbers 806 and 811 are also terms with 770 being the only remaining unknown below them. %e A363491 13 is a term because 2^13 - 5 = 8187 = 3 * 2729 is a semiprime. %t A363491 Select[Range[715],PrimeOmega[2^#-5]==2&] (* _Harvey P. Dale_, Apr 22 2025 *) %o A363491 (PARI) isA363491(k) = bigomega(2^k-5) == 2; \\ _Hugo Pfoertner_, Jun 25 2023 %Y A363491 Cf. A001358, A085724, A363374. %K A363491 nonn %O A363491 1,1 %A A363491 _Kevin P. Thompson_, Jun 05 2023