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 A018825 Numbers that are not the sum of 2 nonzero squares. 18
 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)
 OFFSET 1,2 LINKS T. D. Noe, Table of n, a(n) for n = 1..10000 Index entries for sequences related to sums of squares FORMULA A025426(a(n)) = 0; A063725(a(n)) = 0. - Reinhard Zumkeller, Aug 16 2011 MAPLE 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: A018825 := proc(n) 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: seq(A018825(n), n=1..30) ; # R. J. Mathar, Jul 28 2014 MATHEMATICA 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 *) PROG (Haskell) import Data.List (elemIndices) a018825 n = a018825_list !! (n-1) a018825_list = tail \$ elemIndices 0 a025426_list -- Reinhard Zumkeller, Aug 16 2011 (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 CROSSREFS Cf. A022544, A081324, A000404 (complement), A004431. Sequence in context: A065313 A103566 A258592 * A247779 A243989 A248521 Adjacent sequences: A018822 A018823 A018824 * A018826 A018827 A018828 KEYWORD nonn AUTHOR David W. Wilson STATUS approved

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Last modified June 13 17:32 EDT 2024. Contains 373391 sequences. (Running on oeis4.)