Do you want to calculate SLOPE surface map from DEM? (Almost all GIS software have tool for slope calculation) In ArcGIS you can create SLOPE map by using Spatial Analyst extension (http://resources.arcgis.com/en/help/main/10.1/index.html#//009z000000v2000000).
Or are you looking for alternative methods / algorithms for SLOPE calculation?
thank you very much for your answer. Actually am looking for calculating the Gradient (dh\dl). the slope function available in GIS and I'm using it always.