A sequence begins: a0=1,a1=3,a2=7,a3=15,a4=31a_0 = 1, a_1 = 3, a_2 = 7, a_3 = 15, a_4 = 31a0=1,a1=3,a2=7,a3=15,a4=31. What is the closed form?
an=2n+1−1a_n = 2^{n+1} - 1an=2n+1−1
an=2n+n+1a_n = 2^n + n + 1an=2n+n+1
an=3n+1a_n = 3n + 1an=3n+1
an=n2+1a_n = n^2 + 1an=n2+1