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 A382447 #10 Apr 01 2025 21:30:49 %S A382447 0,2,2,2,2,3,2,1,1,3,3,2,3,2,2,4,6,3,1,3,3,0,1,0,1,1,2,3,2,3,4,2,2,1, %T A382447 5,2,4,2,1,3,4,3,4,2,2,3,2,3,2,3,3,3,4,5,2,2,3,1,3,3,3,4,3,1,0,1,2,1, %U A382447 4,3,3,5,3,3,6,2,3,3,3,2,3,1,1,1,3,1,2,2,2,2,3,3,2,3,2,3,2,3,3,2 %N A382447 Number of positive k <= n such that k*2^n - 1 is prime. %t A382447 a[n_]:=Length[Select[Range[n],PrimeQ[#*2^n-1] &]]; Array[a,100] (* _Stefano Spezia_, Mar 26 2025 *) %o A382447 (Magma) [#[k: k in [1..n] | IsPrime(k*2^n-1)]: n in [1..100]]; %Y A382447 Cf. A002234, A003261, A382119. %Y A382447 Cf. A061411 (indices of 0s), A061414 (indices of 1s). %K A382447 nonn %O A382447 1,2 %A A382447 _Juri-Stepan Gerasimov_, Mar 26 2025