Dr. Smith, having recently finished his PhD, called some of his friends that had just finished their doctorates and masters to comemorate in a trip to his uncle's country house.

During the party, the lights went out for roughly 10 minutes, and a loud noise could be heard. When the lights returned, Dr. Smith was the only one unaccounted for.

They found him dead in his room, with a gun on the floor and a bloodstained note stating, in his own handwriting, Smith's final problem:

There is a number that's 2 and 1, and neither
This number is 0 and 1, and both
This number is not easy to find, but do keep in mind that
To determine this number is to find the identity
This number should tell you who killed me.

The five friends that Smith called were named Ana, Eric, David, Gideon and Tatiana; among the staff only the butler, Mortimer, was present that day.

To give the friends peace of mind until the police solve this case, tell me: "Who killed Dr. Smith?"