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 go out for roughly 10 minutes, a loud noise can be heard. When the lights return, Dr. Smith is the only one unaccounted for.

They find him dead in his room, with a gun on the floor and a bloodstained note saying, 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 5 friends 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, who killed Dr. Smith?

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?"