ELI5?<p>From Wikipedia, which agrees with you...<p>&gt; A unary operation (or function) is idempotent if, whenever it is applied twice to any value, it gives the same result as if it were applied once; i.e., ƒ(ƒ(x)) ≡ ƒ(x). For example, the absolute value function, where abs(abs(x)) ≡ abs(x).<p>So is f(x) = x + 1 idempotent?<p>Under this part: &quot;whenever it is applied twice to any value, it gives the same result as if it were applied once&quot;  it would appear so.
f(2) = 3
f(2) still = 3.<p>But under this part: &quot;ƒ(ƒ(x)) ≡ ƒ(x)&quot;<p>f(f(x)) == f(f(2)) == f(3) = 4<p>Where&#x27;d I go wrong?