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 A336689 #28 Aug 31 2020 12:34:28 %S A336689 15,25,75,125,175,325,341,375,425,561,625,645,1105,1729,1875,1905, %T A336689 2047,2465,3125,3277,4033,4375,4681,5461,6025,6601,8125,8321,8481, %U A336689 8625,9375,10261,10585,10625,12025,12801,15625,15709,15841,16705,16725,18705,25761,29341 %N A336689 Composite numbers k such that the decimal expansion of ((1/2^((k-1)/2))+1)/k or ((1/2^((k-1)/2))-1)/k is finite. %C A336689 If (1/2^((k-1)/2))+-1 divided by k results in a finite decimal number, k is prime or pseudoprime. %C A336689 Euler pseudoprimes: A006970 are a subsequence. %C A336689 If k is a power of 5, both +1 and -1 result in a finite decimal number. %C A336689 A composite integer is part of this list, if and only if %C A336689 (((n-1)!-1)*(1/(2^((n-1)/2)))+1)/n or (((n-1)!-1)*(1/(2^((n-1)/2)))-1)/n results in a finite decimal number. %e A336689 15 is a term because ((1/(2^7))+1)/15 = 0.0671875. %e A336689 9 is not a term because ((1/(2^4))+-1)/9 = 0.11805555... and -0.10416666... . %t A336689 A003592Q[n_] := n/2^IntegerExponent[n, 2]/5^IntegerExponent[n, 5] == 1; seqQ[n_] := CompositeQ[n] && (A003592Q[Denominator[((1/2^((n-1)/2)) + 1)/n]] || A003592Q[ Denominator[((1/2^((n-1)/2)) - 1)/n]]); Select[Range[1, 30000, 2], seqQ] (* _Amiram Eldar_, Jul 31 2020 *) %Y A336689 Cf. A006970 (Euler pseudoprimes, a subsequence), A003592, A334834. %K A336689 nonn,base %O A336689 1,1 %A A336689 _Davide Rotondo_, Jul 31 2020 %E A336689 More terms from _Amiram Eldar_, Jul 31 2020