  For a nonnegative integer $t$, let $c_t$ be the asymptotic density of natural
numbers $n$ for which $s(n + t) \geq s(n)$, where $s(n)$ denotes the sum of
digits of $n$ in base $2$. We prove that $c_t > 1/2$ for $t$ in a set of
asymptotic density $1$, thus giving a partial solution to a conjecture of T. W.
Cusick stating that $c_t > 1/2$ for all t. Interestingly, this problem has
several equivalent formulations, for example that the polynomial $X(X +
1)\cdots(X + t - 1)$ has less than $2^t$ zeros modulo $2^{t+1}$. The proof of
the main result is based on Chebyshev's inequality and the asymptotic analysis
of a trivariate rational function, using methods from analytic combinatorics.
