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This cannot rigorously be described as sigma-pi conjugation since sigma and pi here refer to different defining planes, and interaction between orbitals of different symmetries (with respect to the same defining plane) is forbidden.
Pi is one of the most important numbers in mathematics. It is defined as the ratio of a circle's circumference to its diameter, but it crops up in all sorts of places in mathematics. It is an infinitely long non-recurring decimal number. I'm not going to try to write pi as the Greek letter in this article, because some browsers might show it incorrectly.