Principle of Mathematical Induction

Theorem
Let $\map P n$ be a propositional function depending on $n \in \N$.

Let $n_0 \in \N$ be given. ($n_0$ is often, but not always, zero or one.)

Suppose that:


 * $(1): \quad \map P {n_0}$ is true
 * $(2): \quad \forall k \in \N: k \ge n_0 : \map P k \implies \map P {k + 1}$

Then:


 * $\map P n$ is true for all $n \ge n_0$.

This process is called proof by (mathematical) induction.