fibonacci in javascript (O(log(N)))

fibonacci.js

function multiply_matrix(a, b)
{
        var c = new Array;
        for(y=0;y<a.length;y++){
                c[y] = new Array;
                for(x=0;x<b[0].length;x++){
                        sum = 0;
                        for(i=0;i<a[0].length;i++){
                                sum += a[y][i] * b[i][x];
                        }
                        c[y][x] = sum;
                }
        }
        return c;

}
function pow_matrix(a, n)
{
        if(n == 1){return a;}
        half = pow_matrix(a, ((n-(n%2))/2));
        ret = multiply_matrix(half, half);
        if(n%2){
                ret = multiply_matrix(ret, a);
        }
        return ret;
}
function fibonacci(n){
        // [f(n+2), f(n+1)] = [[1,1], [1,0]] [f(n+1), f(n)];
        // [f(n+2), f(n+1)] = [[1,1], [1,0]]^n [1, 0];
        a = [[1, 1], [1,0]];
        a_n = pow_matrix(a, n);
        start = [[1], [0]];
        end = multiply_matrix(a_n, start);
        return end[1][0];
}

print (fibonacci(arguments[0]));