function M = somma_righe( M , i , lambda , k )
  M( i , : ) =  M( i , : ) + lambda * M( k , : ) ;   % somma righe
  [ n , d ] = rat( lambda ) ;                        % conversione razionale
  fprintf('R%d ← R%d%+d/%d×R%d\n', i, i, n, d, k) ; % output testuale
  disp( M ) ;                                        % output matrice
end