A239435 - OEIS

A239435

Values of n for which the equation x^2 - 25*y^2 = n has integer solutions.

0, 1, 4, 9, 11, 16, 21, 24, 25, 31, 36, 39, 41, 44, 49, 51, 56, 61, 64, 69, 71, 75, 81, 84, 91, 96, 99, 100, 101, 104, 111, 119, 121, 124, 125, 129, 131, 136, 141, 144, 151, 156, 159, 161, 164, 169, 171, 175, 176, 181, 184, 189, 191, 196, 200, 201, 204, 211

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OFFSET

1,3

COMMENTS

This equation is a Pellian equation of the form x^2 - D^2*y^2 = N.

LINKS

Colin Barker, Table of n, a(n) for n = 1..1000

EXAMPLE

21 is in the sequence because the equation x^2 - 25*y^2 = 21 has the solution (X,Y) = (11,2).

PROG

(PARI)

b(n) = sumdiv(n, f, f^2<=n && (n-f^2)%(10*f)==0) \\ See A239434