- Michel Waldschmidt, “Auxiliary functions in transcendental number theory”, Ramanujan J, 20, № 3, 2009,
341 
- Mohammad Reza Yegan, “On the irrationality of π4 and π6”, Journal of Number Theory, 178, 2017,
5 - Jaroslav Hančl, Robert Tijdeman, “On the irrationality of factorial series II”, Journal of Number Theory, 130, № 3, 2010,
595