Linked from
The 9 pages that link to Euler's criterion, each with the reason it gives.
Quadratic reciprocityRelated: It connects residue status to a computable power modulo the prime.
Primality testRelated: Its residue test underlies the Solovay–Strassen test.
Legendre symbolRelated: It computes the symbol by modular exponentiation.
Quadratic residueRelated: It determines quadratic-residue status without listing every square.
Proth's theoremRelated: The test's exponent is half the candidate number minus one.