Examining The Seperability Of Numbers

The seperability of a number is as much a property of a number as its divisibility and deserves, I feel, to be examined.
What do I mean by seperability?
3 can be seperated by 2 and only by 2, as you can see from the below picture.
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infact every number, except 1, can be seperated by the number before it, 2 by 1, 3 by 2; 4 by 3; 5 by 4 etcetera.  1 is the only inseperable number; 1 can be seperated by 0.
No number can be seperated by itself, as you can see from the below picture of an attempt to seperate 2 with 2.
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Seperability and Division when numbers are close to their divisors
Whenever a composite number, except for 9 and some multiples of 9, that is close to its divisor is divided, the seperability of its closest divisor is always the seperability of the number minus its closest divisor.
For example:
The seperability of 2 is 1. The seperability of 1 is 0. 1 -1 = 0
The seperability of 8 is 7. The seperability of 4 is 3. 7 - 4 = 3.
The Seperability of 10 is 9. The seperability of 5 is 4. 9 - 5 is 4.
For some reason, however, this is not true of 9 or multiples of 9 that only have odd numbers as  divisors.
The seperability of 9 is 8 but the seperability of 3 is 2.
The seperability of 27 is 26 but the seperability of 9 is 8.
But 18, because it has 2 as one of its divisors, does conform to the composite number rule.
The seperability of 18 is 17. The seperability of 9 is 8.  17 - 9 = 8.
Another big problem with this composite number rule, furthermore, is that when a number is far from its divisor it doesn't work.
For example, it will work with 100 and its closest divisor 50.
100 has a seperability of 99 and 50 has a seperability of 49 and 99 - 50 =49.
But it doesn't work with 100 and the divisor 10.
100 has a seperability of 99 and 10 has a seperability of 9.