Geodesic
Arithmetic of quaternions and Eisenstein series
Zapiski Nauchnykh Seminarov POMI, Analytical theory of numbers and theory of functions. Part 8, Tome 160 (1987), pp. 82-90 
V. A. Gritsenko. Arithmetic of quaternions and Eisenstein series. Zapiski Nauchnykh Seminarov POMI, Analytical theory of numbers and theory of functions. Part 8, Tome 160 (1987), pp. 82-90. http://geodesic.mathdoc.fr/item/ZNSL_1987_160_a7/
@article{ZNSL_1987_160_a7,
     author = {V. A. Gritsenko},
     title = {Arithmetic of quaternions and {Eisenstein} series},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {82--90},
     year = {1987},
     volume = {160},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1987_160_a7/}
}
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Voir la notice du chapitre de livre provenant de la source Math-Net.Ru

In the paper one computes the Fourier coefficients of the Eisenstein series of the orthogonal group of signature $(1,4)$. The formulas show that the restriction of the Eisenstein series to the “imaginary” axis is a Dirichlet series, whose coefficients are the products of the $L$-series by the number of the representations of the given number as a sum of three squares.