Finding $\sum_{k=0}^{n} \left(_nC_k \mod 2 \right)$ How can I calculate the following expression?
$$\sum_{k=0}^{n} \left(_nC_k \mod 2 \right)$$
To clarify my question, what I am asking is
$$\text{The number of odd coefficients in }(1+x)^n$$

As in any other ring: calculate $\;(1+1)^n$ in $\mathbf Z/2\mathbf Z$. Thee binomial formula is valid in any commutative ring.
