OFFSET
1,1
COMMENTS
If it exists, a(28) > 10^7. - Andrew Howroyd, Aug 17 2019
LINKS
Irving Kaplansky, The first nontrivial genus of positive definite ternary forms, Mathematics of Computation, Vol. 64 (1995): 341-345.
MATHEMATICA
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 *)
PROG
(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)}
select(ok, [1..10000]) \\ Andrew Howroyd, Aug 17 2019
CROSSREFS
KEYWORD
nonn,more
AUTHOR
N. J. A. Sloane, Mar 26 2019
STATUS
approved