use of iodiom "as an" /

[vivcreed]

QUESTION -- he is recognized as an expert in maths field
or
he is recognized as expert in maths field


which one is correct? and why ?

[Willy]

QUESTION -- he is recognized as an expert in maths field
or
he is recognized as expert in maths field


which one is correct? and why ?

First of all, I don't think "As an" is idiom. Most common correct forms of idiom 'as' are

AS X AS Y
AS X SO Y
AS LONG AS
AS MUCH X, AS Y

Your 1st sentence makes me sense not 2nd one. Please correct me.

[RonPurewal]

QUESTION -- he is recognized as an expert in maths field
or
he is recognized as expert in maths field


which one is correct? and why ?

well, "in maths field" isn't proper english, so, technically, both of them are wrong. (you could write just "in math(s)"; note that americans drop the "s". in formal english -- in which "math(s)" is unacceptable -- you could write "in mathematics" or "in the mathematical field".)

in the case of "as an expert" versus "as expert", both are actually acceptable -- because "expert" can be either a noun or an adjective.
i.e., "recognized as a/an NOUN" and "recognized as ADJECTIVE" are both fine.