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A028941
Denominator of X-coordinate of n*P where P is the generator [0,0] for rational points on curve y^2+y = x^3-x.
7
1, 1, 1, 1, 4, 1, 9, 25, 49, 16, 529, 841, 3481, 16641, 98596, 4225, 2337841, 13608721, 67387681, 264517696, 6941055969, 12925188721, 384768368209, 5677664356225, 61935294530404, 49020596163841, 16063784753682169
OFFSET
1,5
COMMENTS
We can take P = P[1] = [x_1, y_1] = [0,0]. Then P[n] = P[1]+P[n-1] = [x_n, y_n] for n >= 2. Sequence gives denominators of the x_n. - N. J. A. Sloane, Jan 27 2022
Squares of terms in A006769 (or A006720).
REFERENCES
G. Everest, A. J. van der Poorten, I. Shparlinski and T. Ward, Recurrence Sequences: Examples and Applications, AMS Monographs, 2003
A. W. Knapp, Elliptic Curves, Princeton 1992, p. 77.
LINKS
B. Mazur, Arithmetic on curves, Bull. Amer. Math. Soc. 14 (1986), 207-259; see p. 225.
FORMULA
This sequence satisfies the quadratic recurrence relation a(n)a(n-6)=-a(n-1)a(n-5)+2a(n-2)a(n-4)+2a(n-3)^2 which is a generalized Somos-6 relation. - Graham Everest (g.everest(AT)uea.ac.uk), Dec 16 2002
P=(0, 0), 2P=(1, 0), if kP=(a, b) then (k+1)P=(a'=(b^2-a^3)/a^2, b'=-1-b*a'/a).
MATHEMATICA
nxt[{a_, b_, c_, d_, e_, f_}]:={b, c, d, e, f, ((c*2e)-f*b+2d^2)/a}; NestList[nxt, {1, 1, 1, 1, 4, 1}, 30][[;; , 1]] (* Harvey P. Dale, Dec 26 2024 *)
PROG
(PARI) - see A028940.
CROSSREFS
KEYWORD
nonn,frac
STATUS
approved