Comment: RSA decrypt(encrypt(P)) = P.
See also exercises/math-examples. With standard variables:
prime(p), prime(q), n = p * q, 2 < p, 2 < q, p ≠ q,
gcd(e, (p - 1) * (q - 1)) = 1, 0 < e, e < n,
≡mod(e * d, 1, (p - 1) * (q - 1)), 0 ≤ d,
C = (P ^ e) |%| n, 0 ≤ P, P < n
⊦ P = (C ^ d) |%| n