We study the fragment of Peano arithmetic formalizing the induction
principle for the class of decidable predicates, $I\Delta1$. We show
that $I\Delta1$ is independent from the set of all true arithmetical
$\Pi2$-sentences. Moreover, we establish the connections between this
theory and some classes of oracle computable functions with
restrictions on the allowed number of queries. We also obtain some
conservation and independence results for parameter free and inference
rule forms of $\Delta1$-induction.
¶ An open problem formulated by J. Paris (see \cite{CloKra,HP}) is
whether $I\Delta1$ proves the corresponding least element principle
for decidable predicates, $L\Delta1$ (or, equivalently, the
$\Sigma1$-collection principle $B\Sigma1$). We reduce this question
to a purely computation-theoretic one.