%I #35 Jun 02 2021 22:09:17
%S 0,1,2,4,12
%N Numbers n such that 2^n-1 is a triangular number (A000217).
%C 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 This sequence maps to the Ramanujan-Nagell squares (8*(2^n - 1) + 1) and is therefore complete. - _Raphie Frank_, Sep 10 2012
%C 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 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/RamanujansSquareEquation.html">Ramanujan's Square Equation</a>
%o (PARI) is(n)=issquare(8<<n-7) \\ _Charles R Greathouse IV_, Sep 07 2012
%Y Cf. A000217, A060728, A038198.
%Y Cf. A076046 (triangular numbers of the form 2^n - 1).
%Y Cf. A060728 (a(n) + 3).
%Y Cf. A038198 (sqrt(8*(2^n - 1)+1)).
%Y Cf. A215797 ((sqrt(8*(2^n - 1)+1) - 1)/2).
%Y Cf. A328129.
%K nonn,fini,full
%O 1,3
%A _V. Raman_, Aug 23 2012
%E Four cross-references to the Ramanujan-Nagell problem added by _Raphie Frank_, Sep 10 2012