Volt-Ampere characteristic of "black box" with a negative resistance
  This problem was given at Third Experimental Physics Olympiad "The day of the resistor", 31 October 2015, Kumanovo, organized by the Regional Society of Physicists of Strumica, Macedonia and the Sofia Branch of the Union of Physicists in Bulgaria. 
Stojan G. Manolev, Vasil G. Yordanov, Nikolay N. Tomchev, Todor M.   Mishonov
physics.ed-ph