The following article is available, free of charge, for download.  Your consideration of the great extent of this research is greatly appreciated. 

  In 1962, Dr. Victor Szebehely studied the famous unsolved
       Restricted Three Body Problem for motion through the solar system
       in determining precise free-fall round-trip interplanetary trajectories
       (such as Earth -- Venus -- Earth) and concluded that it was one of 
       the most difficult problems in celestial mechanics.  See page 55 in
       his article "State of the Art -- 1962 -- Astrodynamics."

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