Hello every body out there ! Let me introduce myself : I am Yoann LE BARS and I am a fresh Boost user. I am french too, so please excuse my bad english. I have got a problem with Boost uBlas, I hope it is the good list to post about it. I have made a little exemple which show what is my problem (it compiles but I found the behaviour very strange) : http://cpp.sourceforge.net/pastebin.php?show=2439 Here is a version which works but do not satisfy me : http://cpp.sourceforge.net/pastebin.php?show=2440 Why does it not satisfy me ? Well, because I am fulling a sparse matrix with an identity matrix, the A matrix and the transposed A matrix : it is very redondant and A can be a very big matrix. That is the reason why I have make the LeastSquareMatrix class. I believe I have done something wrong in it, but what ? Best regards. Yoann LE BARS, alias Le Farfadet Spatial