To qualify for participation in the auctions, banks must be able to offer proof of appropriate collateral in the form of loans to other entities. These can be the public debt of member states, but a fairly wide range of private banking securities are also accepted. The fairly stringent membership requirements for the European Union, especially with regard to sovereign debt as a percentage of each member state's gross domestic product, are designed to insure that assets offered to the bank as collateral are, at least in theory, all equally good, and all equally protected from the risk of inflation.

Answer this question, if possible (if impossible, reply "unanswerable"): What kind of proof is acceptable for participation in auctions?
public debt of member states, but a fairly wide range of private banking securities are also accepted