where b n is approximately 2 n − − 1 / 3 for n > 8 . b n can also be approximated to be 2 n − − 1 / 3 + 4 405 n + 46 25515 n 2 + 131 1148175 n 3 − − 2194697 30690717750 n 4 , for n > 0.36 . It can be shown that b n satisfies γ γ ( 2 n ; b n ) = 1 2 Γ Γ ( 2 n ) , where Γ Γ and γ γ are respectively the Gamma function and lower incomplete Gamma function. Many related expressions, in terms of the surface brightness, also exist.