A $C^1$ prime indicator $\mathcal{ P}\colon\mathbb{ R}\to\mathbb{ R}$ is constructed by applying the Fej\'er identity to the sine-quotient encoder of trial division. For integers $n\ge 2$, $\mathcal P(n)=0$ holds exactly for odd primes; $\mathcal P(2)>0$. For all non-integers $x>1$ one has $\mathcal P(x)>0$. The function is piecewise $C^\infty$ and its second derivative has jumps precisely at the squares $m^2$, with explicit sizes. Replacing the sharp cut-off by a smooth transition yields $C^\infty$ analogues $\mathcal{ P}_\tau$ and $\mathcal{ P}_\sigma$ with integer limits $\mathcal{ P}_\tau(n;\kappa)\to \tau(n)-2$ and $\mathcal{ P}_\sigma(n;\kappa)\to \sigma(n)-n-1$ as $\kappa\to\infty$, obtained from locally uniform convergence of derivative series. For large $\kappa$, numerical evidence indicates companion zeros near odd primes for $\mathcal{ P}_\tau$ and an asymmetric pair for $\mathcal{ P}_\sigma$. No assertion is made beyond integer input, and no statements are claimed about the prime number theorem or zero distributions of $L$-functions. The appendix includes two illustrative prime-counting sums.
Categories: math.NT