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 A215795 #38 Feb 16 2025 08:33:18 %S A215795 0,1,2,4,12 %N A215795 Numbers n such that 2^n-1 is a triangular number (A000217). %C A215795 Aside from a(2), all terms are even. Probably complete; no more terms up to 10^6. - _Charles R Greathouse IV_, Sep 07 2012 %C A215795 This sequence maps to the Ramanujan-Nagell squares (8*(2^n - 1) + 1) and is therefore complete. - _Raphie Frank_, Sep 10 2012 %C A215795 Define equivalence classes on a specified real interval with respect to the symmetric transitive closure of R(x,y) = "x is an integer multiple of y". If any equivalence class is finite (the conditions for which are given in A328129), then a smallest equivalence class has cardinality 1, 2, 4 or 12. - _Peter Munn_, Jun 02 2021 %H A215795 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/RamanujansSquareEquation.html">Ramanujan's Square Equation</a> %t A215795 Select[Range[0,15],OddQ[Sqrt[8(2^#-1)+1]]&] (* _Harvey P. Dale_, Dec 13 2024 *) %o A215795 (PARI) is(n)=issquare(8<<n-7) \\ _Charles R Greathouse IV_, Sep 07 2012 %Y A215795 Cf. A000217, A060728, A038198. %Y A215795 Cf. A076046 (triangular numbers of the form 2^n - 1). %Y A215795 Cf. A060728 (a(n) + 3). %Y A215795 Cf. A038198 (sqrt(8*(2^n - 1)+1)). %Y A215795 Cf. A215797 ((sqrt(8*(2^n - 1)+1) - 1)/2). %Y A215795 Cf. A328129. %K A215795 nonn,fini,full %O A215795 1,3 %A A215795 _V. Raman_, Aug 23 2012 %E A215795 Four cross-references to the Ramanujan-Nagell problem added by _Raphie Frank_, Sep 10 2012