Hi Dan,
The Ritz estimate residual calculations is based on the algorithms described in the following papers as mentioned at the bottom of the eigs documentation page:
1] Lehoucq, R.B. and D.C. Sorensen, "Deflation Techniques for an Implicitly Re-Started Arnoldi Iteration." SIAM J. Matrix Analysis and Applications. Vol. 17, 1996, pp. 789–821.
[2] Sorensen, D.C., "Implicit Application of Polynomial Filters in a k-Step Arnoldi Method." SIAM J. Matrix Analysis and Applications. Vol. 13, 1992, pp. 357–385.