Unrecognized function or variable 'projk'

Error in read>@(x,th)x-projk(x,th/2,opQ,opQt,iter) (line 290)

Psi = @(x,th) x - projk(x,th/2,opQ,opQt,iter);

Error in read (line 300)

xe = Psi(xe+yy-at,lambda)

[Fatou Diabang]

I use IST (iterative shrinkage thresholding algorithm) to solve inverse problem imaging. What function can i remplace by projk for obtain my image at the end?

Thank you for your help.