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A018825
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Numbers that are not the sum of 2 nonzero squares.
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18
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1, 3, 4, 6, 7, 9, 11, 12, 14, 15, 16, 19, 21, 22, 23, 24, 27, 28, 30, 31, 33, 35, 36, 38, 39, 42, 43, 44, 46, 47, 48, 49, 51, 54, 55, 56, 57, 59, 60, 62, 63, 64, 66, 67, 69, 70, 71, 75, 76, 77, 78, 79, 81, 83, 84, 86, 87, 88, 91, 92, 93, 94, 95, 96, 99, 102, 103, 105, 107, 108, 110
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,2
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LINKS
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FORMULA
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MAPLE
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isA000404 := proc(n)
local x, y ;
for x from 1 do
if x^2> n then
return false;
end if;
for y from 1 do
if x^2+y^2 > n then
break;
elif x^2+y^2 = n then
return true;
end if;
end do:
end do:
end proc:
if n = 1 then
1;
else
for a from procname(n-1)+1 do
if not isA000404(a) then
return a;
end if;
end do:
end if;
end proc:
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MATHEMATICA
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q=13; q2=q^2+1; lst={}; Do[Do[z=a^2+b^2; If[z<=q2, AppendTo[lst, z]], {b, a, 1, -1}], {a, q}]; lst; u=Union@lst; Complement[Range[q^2], u] (* Vladimir Joseph Stephan Orlovsky, May 30 2010 *)
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PROG
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(Haskell)
import Data.List (elemIndices)
a018825 n = a018825_list !! (n-1)
a018825_list = tail $ elemIndices 0 a025426_list
(PARI) is(n)=my(f=factor(n), t=prod(i=1, #f~, if(f[i, 1]%4==1, f[i, 2]+1, if(f[i, 2]%2 && f[i, 1]>2, 0, 1)))); if(t!=1, return(!t)); for(k=sqrtint((n-1)\2)+1, sqrtint(n-1), if(issquare(n-k^2), return(0))); 1 \\ Charles R Greathouse IV, Sep 02 2015
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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