HEllo Giwrgo, First of all I would like to thank you for the reply. No the book is not the one you mentioned which I have nad indeed it has an an m-file called "grad.m". The book is called "Applied Numerical Methods Using MATLAB" by Won Young Yang, Wenwu Cao, Tae-Sang. Sorry for the mistake. This code is also available on the web. I do not know what to do. Thanks for the info regarding the grad function. BEcause I thought that Matlab had a afunction called grad (and I did not chech it). You are right. I had a look into the files from this book but none of them was calles grad.m. Thanks BRs Kostas

HEllo Giwrgo, First of all I would like to thank you for the reply. No the book is not the one you mentioned which I have nad indeed it has an an m-file called "grad.m". The book is called "Applied Numerical Methods Using MATLAB" by Won Young Yang, Wenwu Cao, Tae-Sang. Sorry for the mistake. This code is also available on the web. I do not know what to do. Thanks for the info regarding the grad function. BEcause I thought that Matlab had a afunction called grad (and I did not chech it). You are right. I had a look into the files from this book but none of them was calles grad.m. Thanks BRs Kostas