# Left Distributive Law for Natural Numbers

## Theorem

The operation of multiplication is left distributive over addition on the set of natural numbers $\N$:

$\forall x, y, n \in \N_{> 0}: n \times \paren {x + y} = \paren {n \times x} + \paren {n \times y}$

## Proof

Using the axiomatization:

 $(A)$ $:$ $\displaystyle \exists_1 1 \in \N_{> 0}:$ $\displaystyle a \times 1 = a = 1 \times a$ $(B)$ $:$ $\displaystyle \forall a, b \in \N_{> 0}:$ $\displaystyle a \times \paren {b + 1} = \paren {a \times b} + a$ $(C)$ $:$ $\displaystyle \forall a, b \in \N_{> 0}:$ $\displaystyle a + \paren {b + 1} = \paren {a + b} + 1$ $(D)$ $:$ $\displaystyle \forall a \in \N_{> 0}, a \ne 1:$ $\displaystyle \exists_1 b \in \N_{> 0}: a = b + 1$ $(E)$ $:$ $\displaystyle \forall a, b \in \N_{> 0}:$ $\displaystyle$Exactly one of these three holds: $\displaystyle a = b \lor \paren {\exists x \in \N_{> 0}: a + x = b} \lor \paren {\exists y \in \N_{> 0}: a = b + y}$ $(F)$ $:$ $\displaystyle \forall A \subseteq \N_{> 0}:$ $\displaystyle \paren {1 \in A \land \paren {z \in A \implies z + 1 \in A} } \implies A = \N_{> 0}$

Let us cast the proposition in the form:

$\forall a, b, n \in \N_{> 0}: a \times \paren {b + n} = \paren {a \times b} + \paren {a \times n}$

For all $n \in \N_{> 0}$, let $\map P n$ be the proposition:

$\forall a, b \in \N_{> 0}: a \times \paren {b + n} = \paren {a \times b} + \paren {a \times n}$

### Basis for the Induction

$\map P 1$ is the case:

 $\displaystyle a \times \paren {b + 1}$ $=$ $\displaystyle \paren {a \times b} + a$ Axiom $B$ $\displaystyle$ $=$ $\displaystyle \paren {a \times b} + \paren {a \times 1}$ Axiom $A$

and so $\map P 1$ holds.

This is our basis for the induction.

### Induction Hypothesis

Now we need to show that, if $\map P k$ is true, where $k \ge 1$, then it logically follows that $\map P {k + 1}$ is true.

So this is our induction hypothesis:

$\forall a, b \in \N_{> 0}: a \times \paren {b + k} = \paren {a \times b} + \paren {a \times k}$

Then we need to show:

$\forall a, b \in \N_{> 0}: a \times \paren {b + \paren {k + 1} } = \paren {a \times b} + \paren {a \times \paren {k + 1} }$

### Induction Step

This is our induction step:

 $\displaystyle a \times \paren {b + \paren {k + 1} }$ $=$ $\displaystyle a \times \paren {\paren {b + k} + 1}$ Axiom $C$ $\displaystyle$ $=$ $\displaystyle \paren {a \times \paren {b + k} } + a$ Axiom $B$ $\displaystyle$ $=$ $\displaystyle \paren {\paren {a \times b} + \paren {a \times k} } + a$ Induction hypothesis $\displaystyle$ $=$ $\displaystyle \paren {a \times b} + \paren {\paren {a \times k} + a}$ Natural Number Addition is Associative $\displaystyle$ $=$ $\displaystyle \paren {a \times b} + \paren {a \times \paren {k + 1} }$ Axiom $B$

The result follows by the Principle of Mathematical Induction.

$\blacksquare$