In the 18th century, Johann Heinrich Lambert proved that the number Ï (pi) is irrational. That is, it cannot be expressed as a fraction a / b, where a is an integer and b is a non-zero integer.In the 19th century, Charles Hermite found a proof that requires no prerequisite knowledge beyond basic calculus.n the 18th century, Johann Heinrich Lambert proved that the number Ï (pi) is irrational. That is, it cannot be expressed as a fraction a / b, where a is an integer and b is a non-zero integer.