In 2010, Vietnam's total state spending on science and technology amounted to roughly 0.45% of its GDP. Since the dynastic era, Vietnamese scholars have developed many academic fields especially in social sciences and humanities. Vietnam has a millennium-deep legacy of analytical histories, such as the ''Đại Việt sử ký toàn thư'' of Ngô Sĩ Liên. Vietnamese monks, led by the abdicated Emperor Trần Nhân Tông, developed the Trúc Lâm Zen branch of philosophy in the 13th century. Arithmetic and geometry have been widely taught in Vietnam since the 15th century, using the textbook ''Đại thành toán pháp'' by Lương Thế Vinh. Lương Thế Vinh introduced Vietnam to the notion of zero, while Mạc Hiển Tích used the term ''số ẩn'' (Eng: "unknown/secret/hidden number") to refer to negative numbers. Furthermore, Vietnamese scholars produced numerous encyclopaedias, such as Lê Quý Đôn's ''Vân đài loại ngữ''.