st: what is wrong with my nl function in stata?

[张志强 <zhiqiangzhangnk@gmail.com>]

Dear all
I try to estimate a nonlinear lease square function in stata,It need
to estimate a compound summation terms like
lnwage_ij=ko+k1*ln(∑(income*exp(k2*d_ij) , d_ijis the distance between
individual observation i and observation j. My problems is how to deal
with the ∑ in my nl evaluation function. My naive code are as follows,

 the code is as follows:
program nlwage1
version 12
syntax varlist(min=5 max=5) if, at(name)
local lnwage : word 1 of `varlist'
local income : word 2 of `varlist'
local prov1: word 3 of `varlist'
local prov2: word 4 of `varlist'
local prov3:word 5 of `varlist'

tempname k0 k1 k2
scalar `k0' = `at'[1, 1]
scalar `k1' = `at'[1, 2]
scalar `k2' = `at'[1, 3]

tempvar eterm

gen double `eterm'=`income'*exp(`k2'*`prov1')+`income'*exp(`k2'`prov2')+`income'*exp(`k2'*`prov3')
 `if'

replace `lnwage' = `k0' +`k1'*log(`eterm')  `if'

end

nl wage1 @lnwage income prov1 prov2 prov3, parameters(k0 k1 k2)
initial(k0 0.05 k1 0.02 k2 0.03)


it return error

nlwage1 returned 199
verify that nlwage1 is a function evaluator program

What is wrong with my stata code?
I sincerely hope that someone can help me

Zhiqiangzhang

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