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 A004431 Numbers that are the sum of 2 distinct nonzero squares. 65
 5, 10, 13, 17, 20, 25, 26, 29, 34, 37, 40, 41, 45, 50, 52, 53, 58, 61, 65, 68, 73, 74, 80, 82, 85, 89, 90, 97, 100, 101, 104, 106, 109, 113, 116, 117, 122, 125, 130, 136, 137, 145, 146, 148, 149, 153, 157, 160, 164, 169, 170, 173, 178, 180, 181, 185, 193, 194, 197 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers whose prime factorization includes at least one prime congruent to 1 mod 4 and any prime factor congruent to 3 mod 4 has even multiplicity. - Franklin T. Adams-Watters, May 03 2006 Reordering of A055096 by increasing values and without repetition. - Paul Curtz, Sep 08 2008 A063725(a(n)) > 1. - Reinhard Zumkeller, Aug 16 2011 The square of these numbers is also the sum of two nonzero squares, so this sequence is a subsequence of A009003. - Jean-Christophe Hervé, Nov 10 2013 Closed under multiplication. Primitive elements are those with exactly one prime factor congruent to 1 mod 4 with multiplicity one (A230779). - Jean-Christophe Hervé, Nov 10 2013 From Bob Selcoe, Mar 23 2016: (Start) Numbers c such that there is d < c, d >= 1 where c + d and c - d are square. For example, 53 + 28 = 81, 53 - 28 = 25. Given a prime p == 1 mod 4, a term appears if and only if it is of the form p^i, p*2^j or p*k^2 {i,j,k >= 1}, or a product of any combination of these forms. Therefore, the products of any terms to any powers also are terms. For example, p(1) = 5 and p(2) = 13 so term 45 appears because 5*3^2 = 45 and term 416 appears because 13*2^5 = 416; therefore 45 * 416 = 18720 appears, as does 45^3 * 416^11 = 18720^3 * 416^8. Numbers of the form j^2 + 2*j*k + 2*k^2 {j,k >= 1}. (End) Suppose we have a term t = x^2 + y^2. Then s^2*t = (s*x)^2 + (s*y)^2 is a term for any s > 0. Also 2*t = (y+x)^2 + (x-y)^2 is a term. It follows that q*s^2*t is a term for any s > 0 and q=1 or 2. Examples: 2*7^2*26 = 28^2 + 42^2; 6^2*17 = 6^2 + 24^2. - Jerzy R Borysowicz, Aug 11 2017 To find terms up to some upper bound u, we can search for x^2 + y^2 = t where x is odd and y is even. Then we add all numbers of the form 2^m * t <= u and then remove duplicates. - David A. Corneth, Oct 04 2017 LINKS T. D. Noe, Table of n, a(n) for n = 1..10000 EXAMPLE 53 = 7^2 + 2^2, so 53 is in the sequence. MAPLE isA004431 := proc(n)     local a, b ;     for a from 2 do         if a^2>= n then             return false;         end if;         b := n -a^2 ;         if b < 1 then             return false ;         end if;         if issqr(b) then             if ( sqrt(b) <> a ) then                 return true;             end if;         end if;     end do:     return false; end proc: A004431 := proc(n)     option remember ;     local a;     if n = 1 then         5;     else         for a from procname(n-1)+1 do             if isA004431(a) then                 return a;             end if;         end do:     end if; end proc: # R. J. Mathar, Jan 29 2013 MATHEMATICA A004431 = {}; Do[a = 2 m * n; b = m^2 - n^2; c = m^2 + n^2; AppendTo[A004431, c], {m, 100}, {n, m - 1}]; Take[Union@A004431, 63] (* Robert G. Wilson v, May 02 2009 *) Select[Range@ 200, Length[PowersRepresentations[#, 2, 2] /. {{0, _} -> Nothing, {a_, b_} /; a == b -> Nothing}] > 0 &] (* Michael De Vlieger, Mar 24 2016 *) PROG (PARI) isA004431(n)=vecmin((n=factor(n)~%4)[1, ])==1 || return; for( i=1, #n, n[1, i]==3 && n[2, i]%2 & return); 1 \\ M. F. Hasler, Feb 06 2009 (PARI) is(n)=if(n<5, return(0)); my(f=factor(n)%4); if(vecmin(f[, 1])>1, return(0)); for(i=1, #f[, 1], if(f[i, 1]==3 && f[i, 2]%2, return(0))); 1 for(n=1, 1e3, if(is(n), print1(n, ", "))) \\ Altug Alkan, Dec 06 2015 (PARI) upto(n) = {my(res = List(), s); forstep(i=1, sqrtint(n), 2, forstep(j = 2, sqrtint(n - i^2), 2, listput(res, i^2 + j^2))); s = #res; for(i = 1, s, t = res[i]; for(e = 1, logint(n \ res[i], 2), listput(res, t<<=1))); listsort(res, 1); res} \\ David A. Corneth, Oct 04 2017 (Haskell) import Data.List (findIndices) a004431 n = a004431_list !! (n-1) a004431_list = findIndices (> 1) a063725_list -- Reinhard Zumkeller, Aug 16 2011 CROSSREFS Complement of A004439. Cf. A000404, A007692, A009000, A009003, A009177, A024507, A081324, A081325, A118882, A230779. Sequence in context: A242898 A230486 A024507 * A025302 A268379 A221265 Adjacent sequences:  A004428 A004429 A004430 * A004432 A004433 A004434 KEYWORD nonn AUTHOR STATUS approved

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Last modified October 17 16:51 EDT 2019. Contains 328120 sequences. (Running on oeis4.)