Integer as Sum of Three Squares - ProofWiki: no number of the form 8n+7 can be expressed as a sum of three squares.
Legendre: The numbers 3 and 21 can easily be represented as the sum of three squares: 3=1+1+1, 21=16+4+1, but *the product 3*21=63 cannot be so represented since 63 is of the form (8n+7)*.
Can someone explain the starred statement?
Integer as Sum of Three Squares - ProofWiki: no number of the form 8n+7 can be expressed as a sum of three squares.