 It is known to be neither provable nor refutable using the axioms of Zermelo–Fraenkel set theory including the axiom of choice (ZFC), the standard foundation of modern mathematics, in the sense that some models of ZFC satisfy CH, while others violate it.Simple fractions were used by the Egyptians around 1000 BC; the Vedic "Shulba Sutras" ("The rules of chords") in, , include what may be the first "use" of irrational numbers.The concept of irrationality was implicitly accepted by early Indian mathematicians since Manava , who were aware that the square roots of certain numbers such as 2 and 61 could not be exactly determined.