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A139538 Odd numbers of the form x^2+5y^2 not divisible by 5, with both x and y >== 1 0
9, 21, 29, 41, 49, 61, 69, 81, 89, 101, 109, 129, 141, 149, 161, 181, 189, 201, 229, 241, 249, 261, 269, 281, 301, 309, 321, 329, 349, 369, 381, 389, 401, 409, 421, 441, 449, 461, 469, 489, 501, 509, 521, 529, 541, 549, 569, 581, 601, 609, 621, 641, 661, 669 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

All numbers in this sequence are divisible by primes A139513

REFERENCES

Dirichlet & Dedekins Lectures on Number Theory (English Translation 1999) p. 119.

LINKS

Table of n, a(n) for n=1..54.

MATHEMATICA

a = {}; Do[Do[k = x^2 + 5 y^2; If[OddQ[k], If[Mod[k, 5] != 0, AppendTo[a, k]]], {x, 1, 100}], {y, 1, 100}]; Union[a] (*Artur Jasinski*)

With[{upto=1000}, Select[Union[#[[1]]^2+5#[[2]]^2&/@Tuples[Range[Floor[ Sqrt[ upto-5]]], 2]], OddQ[#]&&Mod[#, 5]!=0&&#<=upto&]] (* Harvey P. Dale, Jul 25 2019 *)

CROSSREFS

Sequence in context: A267216 A039289 A045252 * A173460 A110701 A243703

Adjacent sequences:  A139535 A139536 A139537 * A139539 A139540 A139541

KEYWORD

nonn

AUTHOR

Artur Jasinski, Apr 25 2008, Apr 28 2008

EXTENSIONS

Definition clarified by Harvey P. Dale, Jul 25 2019

STATUS

approved

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Last modified January 18 21:06 EST 2021. Contains 340262 sequences. (Running on oeis4.)