
The deck of a graph is the multiset consisting of all unlabelled subgraphs obtained from
by deleting a vertex in all possible ways (counted according to multiplicity).
Conjecture If two graphs on
vertices have the same deck, then they are isomorphic.

See Wikipedia's Reconstruction Conjecture for more on this problem.
Bibliography
*[K] P. J. Kelly, A congruence theorem for trees, Pacific J. Math., 7 (1957), 961–968.
*[U] S. M. Ulam, A collection of mathematical problems, Wiley, New York, 1960.
* indicates original appearance(s) of problem.