The equation that Seven writes on the mirror is known as "Euler's Identity".
There are multiple references to characters from Star Trek: Deep Space Nine (1993). In Picard's hall of skulls are a Ferengi with a Grand Nagus staff (presumably Zek or Rom), Gul Dukat (a primary recurring villain) and General Martok (a major player in the Dominion War who took over the Klingon Empire near the end of the end of the series). There is also a reference to General Sisko, presumably Benjamin Sisko (the lead protagonist).
The First Magistrate (Seven's husband) is played by Jon Jon Briones, father of Isa Briones (Dahj, Soji, etc).
Although it is not mentioned directly, Seven's irritated expression when Picard tells her that Q is involved is an allusion to her prior experience with Q in Q2 (2001).
The animated cat in Agnes' lab is Spot 73. This is an homage to Data's cat Spot.