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Published online by Cambridge University Press: 20 June 2025
The Euler numbers are the coefficients En in the expansion
(1)
Since sec x is an even function and tan x an odd one, this decouples into
(2)
and
(3)
where all three series converge if and E2n + 1 are sometimes called the secant numbers (or secant coefficients) and tangent numbers (or tangent coefficients) respectively.