C (gcc), 88 74 67 bytes
a,b,g;f(n){g=1;for(a=n;a+1&&g;a--)for(b=n;b+1&&g;b--)g=a*a+b*b!=n;}
-14 bytes - thanks Kevin Cruijssen
-7 bytes - by omitting return (using global variable)
C (gcc),
sinvec
- 2k
- 6
- 28