citation Exoo (1989) exhibited an explicit K_5-free 2-coloring of the edges of K_42 (no monochromatic K_5 in either color), giving R(5,5) >= 43. The coloring is published and independently checkable; it is conjectured (McKay & Radziszowski) that R(5,5)=43 exactly, with 656 critical colorings on 42 vertices. Survey DS1 Table Ia lists the lower bound 43.