It's hard to be sure a painting by a well-known artist is authentic. A painting by Vincent Van Gogh was stored in an attic in Norway when it was thought to be by another artist. But the Van Gogh Museum in Amsterdam, using new tools for research and taking more time, has decided the painting is a genuine Van Gogh. It was described in letters by Van Gogh, it was painted in his style, and the paints and canvas were correct. The museum itself had rejected the painting in the 1990s, partly because it was not signed. The painting, called "Sunset at Montmajour" and painted in 1888, was originally sold in 1901, after the painter had died. The painting, owned by a private collector, was on display at the museum until yesterday, Sept. 24, 2013.