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A306968
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Numbers prime to 7 that are not represented by x^2+y^2+7*z^2.
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0
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3, 6, 19, 22, 31, 51, 55, 66, 94, 139, 142, 159, 166, 214, 235, 283, 439, 534, 559, 670, 874, 946, 1726, 2131, 2419, 3559, 4759
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OFFSET
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1,1
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COMMENTS
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LINKS
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MATHEMATICA
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max = 10000;
Table[Table[Table[x^2 + y^2 + 7 z^2, {z, 0, Sqrt[max - x^2 - y^2]/Sqrt[7] // Ceiling}], {y, 0, Sqrt[max - x^2] // Ceiling}], {x, 0, Sqrt[max] // Ceiling}] // Flatten // Union // Select[#, # <= max&]& // Complement[ Select[ Range[max], CoprimeQ[#, 7]&], #]& // Quiet (* Jean-François Alcover, Aug 27 2019 *)
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PROG
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(PARI)
issumsquare(n)={my(f=factor(n)); for(i=1, #f[, 1], if(f[i, 2]%2 && f[i, 1]%4==3, return(0))); 1}
ok(m)={if(m%7, for(i=0, sqrtint(m\7), if(issumsquare(m-7*i^2), return(0))); 1, 0)}
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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STATUS
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approved
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