This is BibTeX, Version 0.99dThe top-level auxiliary file: invopt.aux
The style file: alpha.bst
I found no \bibdata command---while reading file invopt.aux
Warning--I didn't find a database entry for "Neumann84"
Warning--I didn't find a database entry for "Nolet87"
Warning--I didn't find a database entry for "Tarantola87"
Warning--I didn't find a database entry for "Woodhouse84"
(There was 1 error message)