### Abstract

In the mid 80s, Lichtenstein, Pnueli, and Zuck proved a classical
theorem stating that every formula of Past LTL (the extension of LTL
with past operators) is equivalent to a formula of the form
$\bigwedge_{i=1}^n \mathbf{G}\mathbf{F} \varphi_i \vee
\mathbf{F}\mathbf{G} \psi_i$, where $\varphi_i$ and $\psi_i$ contain
only past operators. Some years later, Chang, Manna, and Pnueli built
on this result to derive a similar normal form for LTL. Both
normalisation procedures have a non-elementary worst-case blow-up, and
follow an involved path from formulas to counter-free automata to
star-free regular expressions and back to formulas. We improve on both
points. We present an executable formalisation of a direct and purely
syntactic normalisation procedure for LTL yielding a normal form,
comparable to the one by Chang, Manna, and Pnueli, that has only a
single exponential blow-up.