Let lpf stand for least prime factor. (Assuming gcd(a,b) is 1,) If b is at least as big as a plus lpf(a) plus lpf(b), then you can reach b from a. This is because you can almost always take steps of size 2 to reach the goal. When you can't take a step of size 2, then b must be at most lpf(a) plus lpf(b) plus a, and if b is reachable at all it must be at least that big.

Gerhard "Taking Small Steps Is Easier" Paseman, 2017.09.26.

Let lpf stand for least prime factor. (Assuming gcd(a,b) is 1 and also both are larger than 1,) If b is at least as big as a plus lpf(a) plus lpf(b), then you can reach b from a. This is because you can almost always take steps of size 2 to reach the goal. When you can't take a step of size 2, then b must be at most lpf(a) plus lpf(b) plus a, and if b is reachable at all it must be at least that big.

Gerhard "Taking Small Steps Is Easier" Paseman, 2017.09.26.

Let lpf stand for least prime factor. If b is at least as big as a plus lpf(a) plus lpf(b), then you can reach b from a. This is because you can almost always take steps of size 2 to reach the goal. When you can't take a step of size 2, then b must be at most lpf(a) plus lpf(b) plus a, and if b is reachable at all it must be at least that big.

Gerhard "Taking Small Steps Is Easier" Paseman, 2017.09.26.

Let lpf stand for least prime factor. (Assuming gcd(a,b) is 1,) If b is at least as big as a plus lpf(a) plus lpf(b), then you can reach b from a. This is because you can almost always take steps of size 2 to reach the goal. When you can't take a step of size 2, then b must be at most lpf(a) plus lpf(b) plus a, and if b is reachable at all it must be at least that big.

Gerhard "Taking Small Steps Is Easier" Paseman, 2017.09.26.