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Numbers n such that n^2 is of form x^2+40y^2 with positive x,y.
2

%I #7 Jul 16 2018 09:13:57

%S 7,11,13,14,19,21,22,23,26,28,33,35,37,38,39,41,42,44,46,47,49,52,53,

%T 55,56,57,59,63,65,66,69,70,74,76,77,78,82,84,88,89,91,92,94,95,98,99,

%U 103,104,105,106,110,111,112,114,115,117,118,119,121,123,126,127,130,131

%N Numbers n such that n^2 is of form x^2+40y^2 with positive x,y.

%H Robert Israel, <a href="/A155501/b155501.txt">Table of n, a(n) for n = 1..10000</a>

%p N:= 1000: # to get all terms <= N

%p g:= proc(t) local r; if t^2 > 40*y^2 then return NULL fi;

%p r:= t/2 + 20*y^2/t; if r::integer and r <= N then r fi; end proc:

%p R:= {}:

%p for y from 1 to floor(N/sqrt(40)) do

%p R:= R union map(g, numtheory:-divisors(40*y^2))

%p od:

%p sort(convert(R,list)); # _Robert Israel_, Jul 16 2018

%Y Cf. A155500 (numbers of form x^2+40y^2 with positive x, y).

%K nonn

%O 1,1

%A _Zak Seidov_, Jan 23 2009