  In introductory books about natural numbers, a common kind of assertion -
often left as exercise to the reader - is that certain forms of induction on
$\mathbb{N}$ (regular/ordinary, complete/strong) are equivalent one to each
other and to the well-ordering principle. This means that if P1 and P2 are two
of these principles, then, under all the other usually adopted postulates for
the set of natural numbers (e.g., Peano axioms other than the induction axiom),
P1 implies P2 and vice-versa. In this paper, we shows that, for a reasonable
formalization, based on Peano arithmetic, some of the alleged implications
between these principles hold only if an additional, independent condition is
assumed, namely: every nonzero natural number is a successor. This condition is
a consequence of the regular induction principle, but not of other induction
principles. So, it is necessary to review all currently accepted implications
between induction principles and similar statements in $\mathbb{N}$. From a
list of 9 properties, usually considered induction principles (or equivalent
to), we identify all valid and "almost valid" implications between them, as
well as disprove all invalid implications by counterexamples, which consist of
structures similar to Peano models, but not necessarily satisfying the Peano's
induction axiom, and having an order relation adequately subordinated to the
zero and the successors. Final remarks are made about what can and what cannot
be proved if we weaken the assumptions about the order relation. The aim of
this paper is to shed light on the need for more caution when alleging
equivalence between induction principles for $\mathbb{N}$.
