Buslaev, V. I. “On a Criterion of Rationality for a Series in Orthogonal Polynomials”.
Ukrains’kyi Matematychnyi Zhurnal
, vol. 62, no. 8, Aug. 2010, pp. 1139–1144,
https://umj.imath.kiev.ua/index.php/umj/article/view/2943
.