I am reading Proofs and Confirmations by David Bressoud. On page $150$ is a long excerpt by Richard Askey, from "How can mathematicians and mathematical historians help each other?" There is mentioned
R. Narasimhan told me that he found a definition of "comfortable" series in one of the late volumes of Euler's collected works. For Euler, a comfortable series is a power series whose term ratio is a rational function of $n$. When I asked Narasimhan to give me a specific reference, he was unable to find it again. I will be very pleased to pay $\\\$50$ U.S. for this reference.
When I did a Google Search for this using
Euler "comfortable series",
only a handful of matches were found. Has this Euler reference been