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 A007692 Numbers that are the sum of 2 nonzero squares in 2 or more ways. (Formerly M5299) 15
 50, 65, 85, 125, 130, 145, 170, 185, 200, 205, 221, 250, 260, 265, 290, 305, 325, 338, 340, 365, 370, 377, 410, 425, 442, 445, 450, 481, 485, 493, 500, 505, 520, 530, 533, 545, 565, 578, 580, 585, 610, 625, 629, 650, 680, 685, 689, 697, 725, 730, 740, 745, 754, 765 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A025426(a(n)) > 1. - Reinhard Zumkeller, Aug 16 2011 For the question that is in the link AskNRICH Archive: It is easy to show that (a^2 + b^2)*(c^2 + d^2) = (a*c + b*d)^2 + (a*d - b*c)^2 = (a*d + b*c)^2 + (a*c - b*d)^2. So terms of this sequence can be generated easily. 5 is the least number of the form a^2 + b^2 where a and b distinct positive integers and this is a list sequence. This is the why we observe that there are many terms which are divisible by 5. - Altug Alkan, May 16 2016 Square roots of square terms: {25, 50, 65, 75, 85, 100, 125, 130, 145, 150, 169, 170, 175, 185, 195, 200, 205, 221, 225, 250, 255, 260, 265, 275, 289, 290, 300, 305, ...}. They are also listed by A009177. - Michael De Vlieger, May 16 2016 REFERENCES Ming-Sun Li, Kathryn Robertson, Thomas J. Osler, Abdul Hassen, Christopher S. Simons and Marcus Wright, "On numbers equal to the sum of two squares in more than one way", Mathematics and Computer Education, 43 (2009), 102 - 108. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). D. Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, 125. LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 AskNRICH Archive, Numbers expressible as the sum of 2 squares in more than one way D. J. C. Mackay and S. Mahajan, Numbers that are Sums of Squares in Several Ways G. Xiao, Two squares EXAMPLE 50 is a term since 1^2 + 7^2 and 5^2 + 5^2 equal 50. 25 is not a term since though 3^2 + 4^2 = 25, 25 is square, i.e., 0^2 + 5^2 = 25, leaving it with only one possible sum of 2 nonzero squares. 625 is a term since it is the sum of squares of {0,25}, {7,24}, and {15,20}. MATHEMATICA Select[Range@ 800, Length@ Select[PowersRepresentations[#, 2, 2], First@ # != 0 &] > 1 &] (* Michael De Vlieger, May 16 2016 *) PROG (Haskell) import Data.List (findIndices) a007692 n = a007692_list !! (n-1) a007692_list = findIndices (> 1) a025426_list -- Reinhard Zumkeller, Aug 16 2011 (PARI) isA007692(n)=nb = 0; lim = sqrtint(n); for (x=1, lim, if ((n-x^2 >= x^2) && issquare(n-x^2), nb++); ); nb >= 2; \\ Altug Alkan, May 16 2016 (PARI) is(n)=my(t); if(n<9, return(0)); for(k=sqrtint(n\2-1)+1, sqrtint(n-1), if(issquare(n-k^2)&&t++>1, return(1))); 0 \\ Charles R Greathouse IV, Jun 08 2016 CROSSREFS Subsequence of A001481. A subsequence is A025285 (2 ways). Cf. A004431, A118882, A000404, A018825, A025284 (one way). Sequence in context: A215468 A109552 A206263 * A025285 A092541 A180103 Adjacent sequences:  A007689 A007690 A007691 * A007693 A007694 A007695 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified October 22 13:19 EDT 2019. Contains 328318 sequences. (Running on oeis4.)