Creating polynomial expansion of a variable

[Gino Helbo]

Dear Statalist,

What I want to do is to create a polynomial expansion of a certain degree of two variables k and i in such a way to approximate an unknown function g(k,i). Is there a function in Stata? Doing that manually is quite time consuming and it is dangerous since you can forget some terms easily.

Manually it is (for a polynomia of degree 4): gen phi= i+i*k+i*k*k+i*k*k*k + i*i+ +i*i*k*k+i*i*k+i*i*i+i*i*i*k+k*k*k*k+i*i*i*i

Thank you for your help,

Gino

[Clyde Schechter]

local degree 4

local polynomial

forvalues d = 1/`degree' {  // GENERATE TERMS OF DEGREE d
    forvalues ni = 0/`d' {
        local nk = `d' - `ni'
        local term = `ni'*"*i" + `nk'*"*k"
        local term = substr("`term'", 2, .)
        local polynomial `polynomial' + `term'
    }
}
local polynomial = substr("`polynomial'", 3, .)

display "`polynomial'"

You can change degree at will just by changing the number specified in the first line.

[Gino Helbo]

Thank you Clyde!