Christie’s has announced plans to auction the collection of Arthur and Anita Kahn this fall, a series of 400 works that are valued collectively at $50 million, including 80 works by Alexander Calder, a friend of the family.  “For years and years, I went by [their] apartment and I never knew what was up there, and when I learned, it was, ‘Well, I’ll be darned,’ †said Paul R. Provost, deputy chairman of Christie’s Americas.