Beagle is a software package for phasing genotypes and for imputing
ungenotyped markers.

The Beagle 5.2 genotype imputation method is described in:

B L Browning, Y Zhou, and S R Browning (2018). A one-penny imputed
genome from next generation reference panels. Am J Hum Genet
103(3):338-348. doi:10.1016/j.ajhg.2018.07.015

The most recent reference for Beagle's phasing method is:

S R Browning and B L Browning (2007) Rapid and accurate haplotype
phasing and missing data inference for whole genome association
studies by use of localized haplotype clustering. Am J Hum Genet
81:1084-1097. doi:10.1086/521987