A076046 Ramanujan-Nagell numbers: the triangular numbers (of the form a*(a+1)/2) which are also of the form 2^b - 1.

0, 1, 3, 15, 4095

Offset

1,3

Comments

Ramanujan conjectured and Nagell proved, that the given numbers are the only ones. This sequence is equivalent to A060728, the list of numbers n such that x^2 + 7 = 2^n is soluble, by changing from n to 2^(n-3)-1.

These 5 numbers are therefore the only ones which appear in column k=2 and also in the first subdiagonal of the Stirling2 Sheffer matrix S(n,k) = A048993(n,k). These entries are 0 = S(0, 2) = S(1, 2) = S(1, 0), 1 = S(2, 2) = S(2, 1), 3 = S(3, 2) (intersection of the column k=2 with the first subdiagonal), 15 = S(5, 2) = S(6, 5) and 4095 = S(13, 2) = S(91, 90). The motivation to look into this came from a comment of R. J. Cano on A247024. - Wolfdieter Lang, Oct 16 2014

Named after the Indian mathematician Srinivasa Ramanujan (1887-1920) and the Norwegian mathematician Trygve Nagell (1895-1988). - Amiram Eldar, Jun 22 2021

References

J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", Springer-Verlag, 3rd, 1999. See Chapter 6.

T. Nagell. The Diophantine equation x^2 + 7 = 2^n. Nordisk Mat. Tidskr., Vol. 30 (1948), pp. 62-64; Ark. Math., Vol. 4 (1960), pp. 185-187.

Links

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

Yann Bugeaud and T. N. Shorey. On the number of solutions of the generalized Ramanujan-Nagell equation, J. reine angew. Math., Vol. 539 (2001), pp. 55-74.

Helmut Hasse, Uber eine diophantische Gleichung von Ramanujan-Nagell und ihre Verallgemeinerung, Nagoya Math. J., Vol. 27 (1966), pp. 77-102.

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

Example

4095 can be written as 90*(90+1)/2, but also as 2^12 - 1.

Mathematica

Reap[For[b = 0, b <= 12, b++, If[IntegerQ[(Sqrt[2^(b + 3) - 7] - 1)/2], Sow[2^b - 1]]]][[2, 1]] (* Jean-François Alcover, Jul 05 2017 *)
Select[Accumulate[Range[0, 200]], IntegerQ[Log[2, #+1]]&] (* Harvey P. Dale, Aug 27 2019 *)

Crossrefs

Cf. A060728, A038198, A180445, A215797.

Sequence in context: A012848 A322714 A202621 * A202380 A290610 A134807

Adjacent sequences: A076043 A076044 A076045 * A076047 A076048 A076049

Keyword

fini,full,nonn

Author

Burt Totaro (b.totaro(AT)dpmms.cam.ac.uk), Oct 29 2002

Status

approved