Question. In which book or article George Boole used the notation $\gamma$ for Euler's constant?
Background. Today, Euler's constant is usually denoted by $\gamma$. In 1993 it was found out that the notation $\gamma$ goes back to Carl Anton Bretschneider (1808-1878) (article written in 1835). Now Glaisher writes in "On the history of Euler's constant", 1872:
"Euler’s constant (which throughout this note will be called γ after Mascheroni, De Morgan, &c.) […] It is clearly convenient that the constant should generally be denoted by the same letter. Euler used C and O for it; Legendre, Lindman, &c., C; De Haan A; and Mascheroni, De Morgan, Boole, &c., have written it γ, which is clearly the most suitable, if it is to have a distinctive letter assigned to it."
Unfortunately, Glaisher is wrong. The notation $\gamma$ appears nowhere in the writings of either Euler or Mascheroni. In 2011 it was discovered that De Morgan used the notation $\gamma$ in 1836:
The differential and integral calculus, Baldwin and Craddock, London 1836
See here
Glaisher wrote "De Morgan, Boole, &c." the advanced question is about the "&c."