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 A215797 Numbers n such that n(n+1)/2 + 1 is a power of 2. 3
 0, 1, 2, 5, 90 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS No other terms < 10^6. - T. D. Noe, Aug 25 2012 This sequence maps to the Ramanujan-Nagell squares (8*(n*(n+1)/2)+1) and is therefore complete. - Raphie Frank, Aug 26 2012 All terms in this sequence follow form floor[2^((2*x - 1)/2)]; x = {0, 1, 2, 3, 7}. - Raphie Frank, Mar 03 2013 LINKS Table of n, a(n) for n=1..5. Eric W. Weisstein, MathWorld: Ramanujan's Square Equation FORMULA a(n) = -1 + ceiling[sqrt(2^(A060728(n) - 2) - 1)]. - Raphie Frank, Mar 31 2013 a(n) = (|(((1+i*sqrt(7))/2)^(A060728(n) - 2) + ((1-i*sqrt(7))/2)^(A060728(n) - 2))| - 1)/2. - Raphie Frank, Dec 25 2013 MATHEMATICA Select[Range[0, 1000], IntegerQ[Log[2, 1 + #(#+1)/2]]&] (* T. D. Noe, Aug 25 2012 *) PROG (PARI) for(n=0, 100, if(ispolygonal(2^n-1, 3), print1(sqrtint(2*2^n-2)", "))) \\ Charles R Greathouse IV, Mar 04 2013 CROSSREFS Cf. A060728, A038198 (two references to the Ramanujan-Nagell problem). Sequence in context: A057978 A093308 A319230 * A262325 A290871 A162569 Adjacent sequences: A215794 A215795 A215796 * A215798 A215799 A215800 KEYWORD nonn,fini,full AUTHOR V. Raman, Aug 23 2012 STATUS approved

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Last modified December 1 23:26 EST 2023. Contains 367503 sequences. (Running on oeis4.)