Using the Principle of Mathematical Induction, prove each of the following
different forms of the principle;
(a) Induction with a possibly negative starting point: Suppose that \(S
\subseteq \mathbb{Z}\), that some integer \(n_{0} \in S,\) and that for every \(n
\in \mathbb{Z},\) if \(n \in S\) and \(n \geq n_{0},\) then \(n+1 \in S .\) Then, for
every integer \(n \geq n_{0}\), we have \(n \in S\).
(b) Induction downward: Suppose that \(S \subseteq \mathbb{Z}\), that some
integer \(n_{0} \in S\), and that for every \(n \in \mathbb{Z},\) if \(n \in S\) and
\(n \leq n_{0},\) then \(n-1 \in S .\) Then, for every integer \(n \leq n_{0}\) we
have \(n \in S\)
(c) Finite induction upward: Let \(n_{0}, n_{1} \in \mathbb{Z}, n_{0} \leq
n_{1} .\) Suppose that \(S \subseteq \mathbb{Z}, n_{0} \in S\). and for every \(n
\in \mathbb{Z},\) if \(n \in S, n \geq n_{0},\) and \(n