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A215795 Numbers n such that 2^n-1 is a triangular number (A000217). 5
0, 1, 2, 4, 12 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Aside from a(2), all terms are even. Probably complete; no more terms up to 10^6. - Charles R Greathouse IV, Sep 07 2012

This sequence maps to the Ramanujan-Nagell squares (8*(2^n - 1) + 1) and is therefore complete. - Raphie Frank, Sep 10 2012

LINKS

Table of n, a(n) for n=1..5.

Eric Weisstein's World of Mathematics, Ramanujan's Square Equation

PROG

(PARI) is(n)=issquare(8<<n-7) \\ Charles R Greathouse IV, Sep 07 2012

CROSSREFS

Cf. A000217, A060728, A038198.

Cf. A076046 (triangular numbers of the form 2^n - 1).

Cf. A060728 (a(n) + 3).

Cf. A038198 (sqrt(8*(2^n - 1)+1)).

Cf. A215797 ((sqrt(8*(2^n - 1)+1) - 1)/2).

Sequence in context: A227530 A218129 A156519 * A070314 A075554 A294103

Adjacent sequences:  A215792 A215793 A215794 * A215796 A215797 A215798

KEYWORD

nonn,fini,full

AUTHOR

V. Raman, Aug 23 2012

EXTENSIONS

Four cross-references to the Ramanujan-Nagell problem added by Raphie Frank, Sep 10 2012

STATUS

approved

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Last modified November 14 01:24 EST 2019. Contains 329108 sequences. (Running on oeis4.)