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 A215800 #40 Jan 03 2022 21:10:06 %S A215800 1,3,7,9,13,19,21,27,31,37,39,43,49,57,61,63,67,73,79,81,93,109,111, %T A215800 117,127,129,139,147,151,171,183,189,199,201,217,219,237,243,247,259, %U A215800 277,279,301,313,327,333,351,361,381,387,417,427,433,441,453,457,513,547,549,553,567,589,597,603,613,619,643,651,657 %N A215800 Numbers k such that (2^k+1)/3 can be written in the form a^2 + 3*b^2. %C A215800 These (2^k+1)/3 numbers have no prime factors of the form 2 (mod 3) to an odd power. %H A215800 V. Raman, <a href="/A215800/b215800.txt">Table of n, a(n) for n = 1..84</a> %H A215800 Samuel S. Wagstaff, Jr., The Cunningham Project, <a href="http://homes.cerias.purdue.edu/~ssw/cun/">Factorizations of 2^n-1, for odd n's < 1200.</a> %o A215800 (PARI) for(i=2, 100, a=factorint(2^i+1)~; has=0; for(j=1, #a, if(a[1, j]%3==2&&a[2, j]%2==1, has=1; break)); if(has==0, print(i" -\t"a[1, ]))) %Y A215800 Cf. A000051, A000978, A215801. %K A215800 nonn %O A215800 1,2 %A A215800 _V. Raman_, Aug 23 2012 %E A215800 5 more terms from _V. Raman_, Aug 29 2012