Pivotar[m_,  i_,  j_]  :=  
      ReplacePart[m,  
        m[[i]]  -  (m[[i, j]]/m[[j, j]]) m[[j]], 
        i]

Intercambio[m_,  i_,  j_] :=     ReplacePart[
        ReplacePart[ m, m[[j]],  i], m[[i]],  j]

Proporcional[m_,  i_,  lambda_]  := 
      ReplacePart[m,  m[[i]]*lambda ,  i]

Multiplo[m_,  i_,  
        j_]  :=  -(m[[i, j]]/m[[j, j]])