Are there any references to study the integer solutions (existence and how many) of Diophantine equations like $a^2+b^2=c^2+d^2+2$, $a^2+b^2=c^2+d^2+3$, $a^2+b^2=c^2+d^2+5$...? Thanks a lot.
How many integer solutions of $a^2+b^2=c^2+d^2+n$ are there?
DiophantineStudy
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