This question is primarily mathematical in nature. I have been reading Quantum Field Theory for the Gifted Amateur and I am reading about Feynman’s path integral approach. The definition of the “sum over all paths integral” is given as including a capital pi symbol. For example,
I am familiar with the pi function as a modified gamma function and an extension of the factorial system, but I have never seen it listed with limits. If someone could explain the meaning of this notation or provide an alternate working definition of this integral, I would greatly appreciate.
(the image is not from the book, but it includes the pi symbol I am interested in)