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Impossibility of convergence of a confluent Heun function on the boundary of the disc of convergence

The confluent Heun equation is one of 4 confluent forms of Heun's differential equation in which is the Fuchsian equation of second order with four regular singularities. A confluent Heun function is applicable to diverse areas such as theory of rotating/non-rotating black hole, the gauge theories on thick brane words, Schr$\ddot{\mbox{o}}$dinger equation for hydrogen molecule ion in Stark effect and etc. The confluent Heun function consists of the three term recurrence relation in its power series, and we show that the function is divergent on the boundary of the disc of convergence.