“…The classical Kloosterman sums originated in 1926 in the context of applying the circle method to counting representations of integers by the four-term quadratic form ax 2 + by 2 + cz 2 + dt 2 [16] where c c ≡ 1 (mod c), cc ≡ 1 (mod c ). In 1927, Kloosterman [15] used these sums to estimate Fourier coefficients of modular forms, as did Rademacher in 1937 [23]. Optimal estimates for individual Kloosterman sums were obtained in 1948 by André Weil [32]: |S(a, b, c)| ≤ d(c) (a, b, c) √ c, where d(c) is the number of positive divisors of c and (a, b, c) is the greatest common divisor.…”