2.6.erl

% Direct recursion
product([])->
    1;
product([X|Xs])->X*product(Xs).

% Tail recursion
product(Xs)->product(Xs,1).

product([],P)->
    P;
product([X|Xs],P)->product(Xs,P*X).

% Direct recursion
maximum([X])->
    X;
maximum([X,Y|Xs])->
    maximum([max(X,Y)|Xs]).

% Tail recursion
maximum(Xs)->
    maximum(Xs,0).

maximum([X],M)->
    max(X,M);
maximum([X|Xs],M)->
    maximum(Xs, max(X,M)).