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A020668
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Numbers of the form x^2 + 4*y^2.
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16
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0, 1, 4, 5, 8, 9, 13, 16, 17, 20, 25, 29, 32, 36, 37, 40, 41, 45, 49, 52, 53, 61, 64, 65, 68, 72, 73, 80, 81, 85, 89, 97, 100, 101, 104, 109, 113, 116, 117, 121, 125, 128, 136, 137, 144, 145, 148, 149, 153, 157, 160, 164, 169, 173, 180, 181, 185, 193, 196, 197, 200, 205, 208
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OFFSET
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1,3
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COMMENTS
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x^2 + 4y^2 has discriminant -16.
Numbers that can be expressed as both the sum of two squares and the difference of two squares; the intersection of sequences A001481 and A042965. - T. D. Noe, Feb 05 2003
These are the sum of two squares that are congruent to 0 or 1 (mod 4), and thus that are also the difference of two squares. - Jean-Christophe Hervé, Oct 25 2015
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LINKS
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FORMULA
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MATHEMATICA
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PROG
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(PARI) for(n=0, 1e3, if(if( n<1, n==0, 2 * qfrep([ 1, 0; 0, 4], n)[n]) != 0, print1(n, ", "))) \\ Altug Alkan, Oct 29 2015
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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