Proofs required for the following divisibility rules:

- a|1 implies a= plus or minus 1
- a|b and b|a IF AND ONLY IF (IFF) a= plus or minus b
- a|b IFF -a|b IFF a|(-b) IFF (-a)(-b)
- Let b>0. If a|b implies a is less than or equal to b

Sorry for not inserting mathematical signs, new to the forum so just finding my way around, many thanks.