CEULEN, LUDOLPD VAN (154o-161o), Dutch mathe matician, was born at Hildesheim on Jan. 28, 1540. His name also occurs as Keulen or Collen. He was professor of fortification at Leyden. Van Ceulen's principal work was the calculation of :r to 35 decimal places. This calculation was considered so impor tant that the number is engraved on his tombstone at Leyden and it is often called Ludolph's number. He wrote Die arithme tische en geometrische Fondamenten (1615) and Van den Circkel (1S96), a second edition of which was edited in 1615 by van Ceulen's widow, who had helped him in his calculations of 7r; a third edition in Latin was published in 1619. He died on Dec. 31, 161o, at Leyden.