---
name: "gradls"
section: "3m"
platform: "linux"
package: "iraf"
version: "2.11.3-2"
release: "Debian 3.0 (Woody)"
platforms: ["linux"]
url: "https://new.carta.tech/gradls"
---

# gradls(3m)

## NAME

gradls

## DESCRIPTION

subroutine gradls.f

source\
   Bevington, pages 220-222.

purpose\
   make gradient-search least-squares fit to data with a\
     specified function which is not linear in coefficients

usage \
   call gradls (x, y, sigmay, npts, nterms, mode, a, deltaa,\
     yfit, chisqr)

description of parameters\
   x      - array of data points for independent variable\
   y      - array of data points for dependent variable\
   sigmay - array of standard deviations for y data points\
   npts   - number of pairs of data points\
   nterms - number of parameters\
   mode   - determines method of weighting least-squares fit\
            \+1 (instrumental) weight(i) = 1./sigmay(i)\*\*2\
             0 (no weighting) weight(i) = 1.\
            \-1 (statistical)  weight(i) = 1./y(i)\
   a      - array of parameters\
   deltaa - array of increments for parameters a\
   yfit   - array of calculated values of y\
   chisqr - reduced chi square for fit

subroutines and function subprograms required \
   functn (x, i, a)\
     evaluates the fitting function for the ith term\
   fchisq (y, sigmay, npts, nfree, mode, yfit) \
     evaluates reduced chi squared for fit to data

comments\
   valid for nterms up to 10