# How can i express ||(a,b)||2?

hello,everyone.do you know how can i express ||(a,b)||2 in matalb?(a is number,b is matrix)
i know ||(a)||2 is norm(a,2),but how can i express ||(a,b)||2?

norm squared can be entered as `square_pos(norm(...))`

Edit: On 2nd thought, it looks like I have misinterpreted the question. I don’t know what you’re asking. Pretend `a` and `b` are just numerical MATLAB variables (not CVX variables or expressions). What would the MATLAB expression be to evaluate what you’re interested in? If `a` is a scalar and `b` is a matrix, what (mathematical object) is `(a,b)` ? Perhaps you should give an example for some small matrix `b`.

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a,b are variable,but it is different to express them,when they are others??

I don’t understand your last post.

Give an example where `a` and `b` are numbers,and show us what `(a,b)` is supposed to be in that example.

variable a(a is matrix of 1*1),variable b(b is matrix of 20 1)

In this case, `b` is a special kind of matrix called a vector. Presuming that `b` is always a column vector, as would be declared in CVX by
`variable b(n)`
where n is an integer, such as 20 in your case, then the code would be
`norm([a;b])`
or
`norm([a;b],2)`
because the default norm in MATLAB and CVX is the 2 norm.

I suggest you spend some time brushing up on the basics of MATLAB, such as concatenation - that will help you a lot in using CVX.