1,2

x^2 - 2y^2 has discriminant 8. - N. J. A. Sloane, May 30 2014

A positive number n is representable in the form x^2 - 2y^2 iff every prime p == 3 or 5 (mod 8) dividing n occurs to an even power.

Indices of nonzero terms in expansion of Dirichlet series Product_p (1-(Kronecker(m,p)+1)*p^(-s)+Kronecker(m,p)*p^(-2s))^(-1) for m=2 (A035185). [amended by Georg Fischer, Sep 03 2020]

Also positive numbers of the form 2x^2 - y^2. If x^2 - 2y^2 = n, 2(x+y)^2 - (x+2y)^2 = n. - Franklin T. Adams-Watters, Nov 09 2009

Except 2, prime numbers in this sequence have the form p=8k+-1. According to the first comment, prime factors of the forms (8k+-3),(8k+-5) occur in x^2 - 2y^2 in even powers. If x^2 - 2y^2 is a prime number, those powers must be 0. Only factors 8k+-1 remain. Example: 137=8*17+1. - Jerzy R Borysowicz, Nov 04 2015

The product of any two terms of the sequence is a term too. A proof follows from the identity: (a^2-2b^2)(c^2-2d^2) = (2bd+ac)^2 - 2(ad+bc)^2. Example: 127*175 has form x^2-2y^2, with x=9335, y=6600. - Jerzy R Borysowicz, Nov 28 2015

Primitive terms (not a product of earlier terms that are greater than 1 in the sequence) are A055673 except 1. - Charles R Greathouse IV, Sep 10 2016

Positive numbers of the form u^2 + 2uv - v^2. - Thomas Ordowski, Feb 17 2017

For integer numbers z, a, k and z^2+a^2>0, k>=0: z^(4k) + a^4 is in A035251 because z^(4k) + a^4 = (z^(2k) + a^2)^2 - 2(a*z^k)^2. Assume 0^0 = 1. Examples: 3^4 + 1^4 = 82, 3^8+4^4=6817. - Jerzy R Borysowicz, Mar 09 2017

Numbers that are the difference between two legs of a Pythagorean right triangle. - Michael Somos, Apr 02 2017

T. D. Noe, Table of n, a(n) for n = 1..1000

Wolfdieter Lang, On Positive Integer Descartes-Steiner Curvature Quintuplets, arXiv:2503.08631 [math.NT], 2025. See p. 23.

Keith Matthews, Thue's theorem and the diophantine equation x^2-D*y^2=+-N, Math. Comp. 71 (239) (2002) 1281-1286.

Keith Matthews, Solving the diophantine equation x^2-D*y^2=N, D>0, (2016).

Sci.math, General Pell equation: x^2 - N*y^2 = D, 1998

Sci.math, General Pell equation: x^2 - N*y^2 = D, 1998 [Edited and cached copy]

N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references)

The (x,y) pairs, with minimum x, that solve the equation are (1,0), (2,1), (2,0), (3,1), (4,2), (3,0), (4,1), (4,0), (5,2), (6,3), (5,1), (5,0), (6,2), (7,3), (8,4), (6,1), (6,0), (7,2), (8,3), (7,1), (7,0), (10,5), (8,2), ... If the positive number is a perfect square, y=0 yields a trivial solution. - R. J. Mathar, Sep 10 2016

filter:= proc(n) local F;

F:= select(t -> t[1] mod 8 = 3 or t[1] mod 8 = 5, ifactors(n)[2]);

map(t -> t[2], F)::list(even);

end proc:

select(filter, [$1..1000]); # Robert Israel, Dec 01 2015

Reap[For[n = 1, n < 200, n++, r = Reduce[x^2 - 2 y^2 == n, {x, y}, Integers]; If[r =!= False, Sow[n]]]][[2, 1]] (* Jean-François Alcover, Oct 31 2016 *)

(PARI) select(x -> x, direuler(p=2, 201, 1/(1-(kronecker(2, p)*(X-X^2))-X)), 1) \\ Fixed by Andrei Zabolotskii, Jul 30 2020

(PARI) {a(n) = my(m, c); if( n<1, 0, c=0; m=0; while( c<n, m++; if( sum(i=0, sqrtint(m\2), issquare(m+2*i^2)), c++)); m)}; /* Michael Somos, Aug 17 2006 */

(PARI) is(n)=#bnfisintnorm(bnfinit(z^2-2), n) \\ Ralf Stephan, Oct 14 2013

(Python)

from itertools import count, islice

from sympy import factorint

def A035251_gen(): # generator of terms

return filter(lambda n:all(not((2 < p & 7 < 7) and e & 1) for p, e in factorint(n).items()), count(1))

A035251_list = list(islice(A035251_gen(), 30)) # Chai Wah Wu, Jun 28 2022

Cf. A035185, A042965, A001481, A000047.

Primes: A038873.

Complement of A232531. - Thomas Ordowski and Altug Alkan, Feb 09 2017

Sequence in context: A116724 A391346 A391742 * A391348 A141401 A132604

Adjacent sequences: A035248 A035249 A035250 * A035252 A035253 A035254

Better description from Sharon Sela (sharonsela(AT)hotmail.com), Mar 10 2002

approved