R-torsion and the Laplacian on Riemannian manifolds by Daniel Ray and Isadore Singer (1971)

Original paper scan: www.ms.uky.edu/~hislop/papers/ray-singer1.pdf

Advances in Mathematics 7(2), pages 145–210. Definition 1.6, page 149, gives analytic torsion by spectral zeta derivatives; Section 2 proves metric independence for acyclic closed-manifold coefficients; Section 3 handles nonzero homology and boundary data. DOI: doi.org/10.1016/0001-8708(71)90045-4

The original prints D. B. Ray and I. M. Singer. Their first names Daniel Ray and Isadore Singer are explicitly identified in the opening of the author-hosted historical article math.berkeley.edu/~lott/torsion.pdf .