login
A239365
Numbers n such that 10*n^2+4 is a square.
2
12, 456, 17316, 657552, 24969660, 948189528, 36006232404, 1367288641824, 51920962156908, 1971629273320680, 74869991424028932, 2843088044839778736, 107962475712487563036, 4099730989029687616632, 155681815107415641868980, 5911809243092764703404608
OFFSET
1,1
COMMENTS
Values of y satisfying the Pellian equation x^2 - 10*y^2 = 4.
FORMULA
a(n) = 2*A078987(n-1).
a(n) = 38*a(n-1)-a(n-2).
G.f.: 12*x / (x^2-38*x+1).
EXAMPLE
456 is in the sequence because 10*456^2+4 = 2079364 = 1442^2.
MATHEMATICA
LinearRecurrence[{38, -1}, {12, 456}, 20] (* Harvey P. Dale, Aug 04 2022 *)
PROG
(PARI) Vec(12/(x^2-38*x+1) + O(x^100))
CROSSREFS
Sequence in context: A295413 A202799 A121348 * A277262 A226358 A225796
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Mar 17 2014
STATUS
approved