Painting has continued to be a prominent form of artistic expression in Danish culture, inspired by and also influencing major international trends in this area. These include impressionism and the modernist styles of expressionism, abstract painting and surrealism. While international co-operation and activity has almost always been essential to the Danish artistic community, influential art collectives with a firm Danish base includes De Tretten (1909–1912), Linien (1930s and 1940s), COBRA (avant-garde movement) (1948–51), Fluxus (1960s and 1970s), Junge Wilde (1980s) and more recently Superflex (founded in 1993). Most Danish painters of modern times have also been very active with other forms of artistic expressions, such as sculpting, ceramics, art installations, activism, film and experimental architecture. Notable Danish painters from modern times representing various art movements include Theodor Philipsen (1840–1920, impressionism and naturalism), Anna Klindt Sørensen (1899–1985, expressionism), Franciska Clausen (1899–1986, Neue Sachlichkeit, cubism, surrealism and others), Henry Heerup (1907–1993, naivism), Robert Jacobsen (1912–1993, abstract painting), Carl Henning Pedersen (1913–2007, abstract painting), Asger Jorn (1914–1973, Situationist, abstract painting), Bjørn Wiinblad (1918–2006, art deco, orientalism), Per Kirkeby (b. 1938, neo-expressionism, abstract painting), Per Arnoldi (b. 1941, pop art), Michael Kvium (b. 1955, neo-surrealism) and Simone Aaberg Kærn (b. 1969, superrealism).. What two Danish art collectives were active in the 1940s?