= Ising-chain decimation constant
{c}
{title2=$A=2\sqrt{\cosh2K}$}
For a periodic even chain, eliminating alternate spins gives $Z_N(K)=A^{N/2}Z_{N/2}(K')$. Thus dimensionless <free energy> per original spin obeys $f(K)=-\tfrac12\log A+\tfrac12 f(K')$. The exact solution $f(K)=-\log(2\cosh K)$ satisfies this relation. The multiplicative constant cancels from normalized retained-spin <probabilities> but records eliminated-spin <entropy>; discarding it loses thermodynamics even if the interaction recursion is correct.
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