1221555591 has 24 divisors (see below), whose sum is σ = 1813543680. Its totient is φ = 792030096.

The previous prime is 1221555443. The next prime is 1221555617. The reversal of 1221555591 is 1955551221.

It is a happy number.

1221555591 is a `hidden beast` number, since 1 + 2 + 2 + 15 + 55 + 591 = 666.

It is not a de Polignac number, because 1221555591 - 2⁹ = 1221555079 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1221555191) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 34816 + ... + 60458.

It is an arithmetic number, because the mean of its divisors is an integer number (75564320).

Almost surely, 2^1221555591 is an apocalyptic number.

1221555591 is a deficient number, since it is larger than the sum of its proper divisors (591988089).

1221555591 is a wasteful number, since it uses less digits than its factorization.

1221555591 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 25795 (or 25792 counting only the distinct ones).

The product of its digits is 22500, while the sum is 36.

The square root of 1221555591 is about 34950.7595196442. The cubic root of 1221555591 is about 1068.9836889623.

The spelling of 1221555591 in words is "one billion, two hundred twenty-one million, five hundred fifty-five thousand, five hundred ninety-one".