The purpose of this paper is to give a partial answer to the question: How much is the induction axiom weakened if it is applied only to sentences with no bound variables? It is well known that for the full Peano arithmetic this is a weakening ([1] p. 90). We consider Peano arithmetic without multiplication, and give a full answer to the question. It turns out that only four new theorems can be proved from the weakened induction axiom; i.e., all further consequences of this axiom are derivable from these four.

We consider a system T formulated within the first-order predicate calculus with equality. The system contains the constant 0 and the three function symbols S (successor), P (predecessor), and +. The non-logical axioms are:

A sentence is open if it contains no bound variables. We obtain the system TI from T by adding the rule of inference:

(I) If A(x) is an open sentence, infer A(x) from A(0) and A(x) ⊃ A(Sx).

The following open sentences are easily proved in TI:

The system formed by adding (B1) - (B4) to T is called T′.

We abbreviate SS … Sx, where S occurs n times, to Snx. Similarly, we abbreviate (… (x+x)+ …) + x, where x occurs n times, to nx. A term of the form n1x1 + … + nkXk + Sp0 is called simple. An equation between simple terms is called a simple equation.