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A218800
Number of nonnegative integer solutions to x^2 + 2y^2 = (3n)^2.
2
1, 2, 2, 3, 2, 2, 3, 2, 2, 4, 2, 5, 3, 2, 2, 3, 2, 5, 4, 5, 2, 3, 5, 2, 3, 2, 2, 5, 2, 2, 3, 2, 2, 8, 5, 2, 4, 2, 5, 3, 2, 5, 3, 5, 5, 4, 2, 2, 3, 2, 2, 8, 2, 2, 5, 5, 2, 8, 2, 5, 3, 2, 2, 4, 2, 2, 8, 5, 5, 3, 2, 2, 4, 5, 2, 3, 5, 5, 3, 2, 2, 6, 5, 5, 3, 5, 5
OFFSET
0,2
COMMENTS
For n > 0, a(n) > 1 since n^2 + 2(2n)^2 = (3n)^2 and (3n)^2 + 2*0^2 = (3n)^2.
a(3k) > 2 as we also have (7k)^2 + 2*(4k)^2 = 81k^2 =
(9k)^2 = (3*3k)^2.
EXAMPLE
a(2) = 2 because we have 6^2 + 2*0^2 = 6^2 and 2^2 + 2*4^2 = 6^2 and no others.
PROG
(JavaScript)
for (i=0; i<200; i+=3) {
d=0; e=0;
for (a=0; a<=i; a++)
for (b=0; b<=i; b++) {
t1=Math.pow(a, 2)+2*Math.pow(b, 2);
t2=Math.pow(i, 2);
if (t1<t2) d++;
if (t1<=t2) e++;
}
document.write((e-d)+", ");
}
CROSSREFS
Cf. A218799.
Sequence in context: A152727 A087159 A366922 * A062502 A141242 A130451
KEYWORD
nonn
AUTHOR
Jon Perry, Nov 06 2012
STATUS
approved