Necessary and sufficient

In mathematics, the phrase "necessary and sufficient" is frequently used, for instance, in the formulation of theorems, in the text of proofs when a step has to be justified, or when an alternative version for a definition is given.

To say that a statement is "necessary and sufficient" to another statement means that the statements are either both true or both false.

Another phrase with the same meaning is "if and only if".

Necessary and sufficient

A statement A is (a) necessary and sufficient (condition) A statement A is necessary and sufficient

                 is a necessary and sufficient condition

for another statement B if it is both a necessary condition and a sufficient condition for B, i.e., if the following two propositions both are true:

• A is (a) necessary (condition) for B,

Necessary

The statement

• A is a necessary condition for B,

       (or shorter: is necessary for) B,

means precisely the same as each of the following statements:

• B is false whenever A does not hold, or, equivalently.
• B implies A.
• If A is false then B cannot be true

Sufficient

• A is (a) sufficient (condition) for B,
• A is a sufficient condition for

       (or shorter: is sufficient for) B,

means precisely the same as each of the following statements:

• B holds whenever A is true.
• B holds whenever A is true.
• A implies B.

Example

For a sequence of positive real numbers to converge against a limit

• it is necessary that the sequence is bounded
• it is sufficient that the sequence is monotone decreasing
• it is necessary and sufficient that it is a Cauchy sequence.

A sequence $(a_{n}),\ 0\leq a_{n}\in textrmR$ $\lim _{n\to \infty }a_{n}=a\Rightarrow a_{n}\ {\text{bounded}}$ $\lim _{n\to \infty }a_{n}=a\Leftarrow a_{n}\ {\text{monotone decreasing}}$ $\lim _{n\to \infty }a_{n}=a\Leftrightarrow a_{n}\ {\text{is a Cauchy sequence}}$

$\lim _{n\to \infty }a_{n}=a\Rightarrow a_{n}<C\in {\textrm {R}}$ $\lim _{n\to \infty }a_{n}=a\Leftarrow (\exists C)(\forall n)a_{n}<C\in {\textrm {R}}$ $\lim _{n\to \infty }a_{n}=a\Leftarrow (\forall \epsilon >0)(\exists N\in {\textrm {N}})$