Brun, V. and Knudsen, F.F. 1972. On the Possibility of Finding Certain Criteria for the Irrationality of a Number Defined as a Limit of a Sequence of Rational Numbers. MATHEMATICA SCANDINAVICA. 31, (Jun. 1972), 231–236. DOI:https://doi.org/10.7146/math.scand.a-11429.