Sginum White Papers
Signum white paper, incorporating the best from several predecessors
As a coin resulting from several forks of successive forks of Bitcoin, Signum retains many traits inherited from preceding coins. As you may know from reading the history of Signum, Signum is a code fork based on an extensive modification of NXT, which itself is a fork of Bitcoin. As such, there are several relevant white papers.
Second is the NXT community white paper: With the NXT fork from NXT, Signum removed proof-of-stake and replaced it with proof-of-capacity. A concept similar to proof-of-capacity is referred to as proof-of-space (no active coins). The proof of space white paper by Dziembowski, Faust, Kolmogorov, and Pietrzak is nevertheless relevant. Based on this paper, a proof-of-space cryptocurrency called SpaceMint was proposed in a white paper by Sunoo Park, Kwon, Fuchsbauer, Pietrzak, et al. Both of these papers are only mentioned as a loose reference. The SpaceMint paper specifically references Signum.