Exponents of Primes in Prime Decomposition are Less iff Divisor

Theorem
Let $a, b \in \Z_{>0}$.

Then $a \mathop \backslash b$ iff:
 * $(1): \quad$ every prime $p_i$ in the prime decomposition of $a$ appears in the prime decomposition of $b$

and:
 * $(2): \quad$ the exponent of each $p_i$ in $a$ is less than or equal to its exponent in $b$.

Proof
Let $a, b \in \Z_{>0}$.

Let their prime decomposition be:


 * $a = p_1^{k_1} p_2^{k_2} \ldots p_n^{k_n}$
 * $b = q_1^{l_1} q_2^{l_2} \ldots q_n^{l_n}$

Necessary Condition
Let:
 * $(1): \quad$ prime in the prime decomposition of $a$ appear in the prime decomposition of $b$

and:
 * $(2): \quad$ its exponent in $a$ be less than or equal to its exponent in $b$.

Then:

where:
 * $k_1 \le l_1, k_2 \le l_2, \ldots, k_r \le l_r, r \le s$

Thus:
 * $d = p_1^{l_1-k_1} p_2^{l_2-k_2} \ldots p_r^{l_r-k_r} \in \Z$ and $b = a d$

So $a \mathop \backslash b$.

Sufficient Condition
Let $a \mathop \backslash b$.

Let $a = p_1^{k_1} p_2^{k_2} \ldots p_r^{k_r}$ be the prime decomposition of $a$.

Then:
 * $\forall i \in \N_r: p_i^{k_i} \mathop \backslash a$

Hence by Divides is Partial Ordering on Positive Integers each $p_i^{k_i}$ also divides $b$.

Thus:
 * $\exists c \in \Z: b = p_i^{k_i} c$

The prime decomposition of $b$ is therefore:


 * $b = p_i^{k_i} ($ prime decomposition of $c)$

which may need to be rearranged.

So $p_i$ must occur in the prime decomposition of $b$ with an exponent at least as big as $k_i$.

The result follows.