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\(B'''(x) = 1/(4*\tanh(x/2)*\sinh(x/2)^2) - 2/x^3\) used Taylor series expansion for x near 0 for stability
bftprimeprimeprime(x)
\(B'''(x) = 1/(4*\tanh(x/2)*\sinh(x/2)^2) - 2/x^3\)
input vector