 According to Raymond Ayoub, the fact that the divergent zeta series is not Abel summable prevented Euler from using the zeta function as freely as the eta function, and he "could not have attached a meaning" to the series.Other authors have credited Euler with the sum, suggesting that Euler would have extended the relationship between the zeta and eta functions to negative integers.In the primary literature, the series is mentioned in Euler's 1760 publication "De seriebus divergentibus" alongside the divergent geometric series .