LPF - signal values unaffected at specific times
What you probably are looking for are Nyquist/$M$th-band filters. The impulse response of these have the following property (assuming a non-causal impulse response centered around tap 0 for ease of exposition)

$h_n = \left\{ \begin{matrix} \frac{1}{M} &amp; n=0\\ 0 &amp; n = kM, k=\pm 1, \pm 2, \dots\end{matrix} \right.$

This means that every $M$th sample is untouched (the $1/M$ term is for scaling purposes). Often it is said that these filters have zero inter symbol interference, assuming that a symbol arrived every $M$th sample.