William H. Press, Brian P. Flannery, Saul A. Teukolsky, William T. Vetterling, Numerical Recipes in C: The Art of Scientific Computing, Cambridge University Press, 1992.
The columns T (temperature), C(heat capacity), and d2C/dT2 (i.e., the second derivative with temperature) correspond to arrays xa, ya, and y2a in the implementation of the spline fit in section 3.3 of the reference (page 116). This information is also available in the 3rd edition (2007) of the book on page 120.
The heat capacities are assumed to follow a T cubed form below the lowest temperature reported. The original
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