

A035251


Positive numbers of the form x^2  2y^2 with integers x, y.


14



1, 2, 4, 7, 8, 9, 14, 16, 17, 18, 23, 25, 28, 31, 32, 34, 36, 41, 46, 47, 49, 50, 56, 62, 63, 64, 68, 71, 72, 73, 79, 81, 82, 89, 92, 94, 97, 98, 100, 103, 112, 113, 119, 121, 124, 126, 127, 128, 136, 137, 142, 144, 146, 151, 153, 158, 161, 162, 164, 167, 169, 175, 178
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OFFSET

1,2


COMMENTS

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. AdamsWatters, 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^22b^2)(c^22d^2) = (2bd+ac)^2  2(ad+bc)^2. Example: 127*175 has form x^22y^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 reduced Pythagorean right triangle.  Michael Somos, Apr 02 2017


LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000
K. Matthews, Thue's theorem and the diophantine equation x^2D*y^2=+N, Math. Comp. 71 (239) (2002) 12811286.
K. Matthews, Solving the diophantine equation x^2D*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)


EXAMPLE

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


MAPLE

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


MATHEMATICA

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]] (* JeanFrançois Alcover, Oct 31 2016 *)


PROG

(PARI) select(x > x, direuler(p=2, 201, 1/(1(kronecker(2, p)*(XX^2))X)), 1) \\ Fixed by Andrey Zabolotskiy, 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^22), n) \\ Ralf Stephan, Oct 14 2013


CROSSREFS

Cf. A035185, A042965, A001481, A000047.
Primes: A038873.
Complement of A232531.  Thomas Ordowski and Altug Alkan, Feb 09 2017
Sequence in context: A190244 A182636 A116724 * A141401 A132604 A013153
Adjacent sequences: A035248 A035249 A035250 * A035252 A035253 A035254


KEYWORD

nonn


AUTHOR

N. J. A. Sloane


EXTENSIONS

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


STATUS

approved



