Because the range of m < 4 is smaller than that of m≤4, a small range can lead to a large range, so the former is a sufficient condition for the latter, while a large range cannot lead to a small range, so m < 4 is a necessary and sufficient condition for m≤4.

Where there are things, there must be things; If there is a thing case B, there is not necessarily a thing case A. A is the necessary and sufficient condition of B, that is, the necessary and sufficient condition. Necessary and sufficient conditions for a to be B←→B to be a.

Suppose a is the condition and b is the conclusion:

(1) If B can be deduced from A and A can be deduced from B, then A is the necessary and sufficient condition of B (A=B).

(2) If B can be derived from A and A cannot be derived from B, then A is a necessary and sufficient condition for B (A? B) yes.

(3) If B cannot be derived from A and A can be derived from B, then A is a necessary and sufficient condition for B (B? Answer.

(4) If A cannot deduce B and B cannot deduce A, then A is an insufficient and unnecessary condition of B (A￠B and B￠A).

for instance

Example: It is known that P is a necessary and sufficient condition for R, S is a necessary condition for R, and Q is a necessary condition for S. What is the condition of P for Q?

Solution: R is derived from condition P, R is derived from S, S is derived from Q, and R cannot derive P, so P is a necessary and sufficient condition of Q.