1221865721 has 4 divisors (see below), whose sum is σ = 1247862912. Its totient is φ = 1195868532.

The previous prime is 1221865717. The next prime is 1221865753. The reversal of 1221865721 is 1275681221.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 1275681221 = 53 ⋅24069457.

It is a cyclic number.

It is not a de Polignac number, because 1221865721 - 2² = 1221865717 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1221865321) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 12998525 + ... + 12998618.

It is an arithmetic number, because the mean of its divisors is an integer number (311965728).

Almost surely, 2^1221865721 is an apocalyptic number.

It is an amenable number.

1221865721 is a deficient number, since it is larger than the sum of its proper divisors (25997191).

1221865721 is an equidigital number, since it uses as much as digits as its factorization.

1221865721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 25997190.

The product of its digits is 13440, while the sum is 35.

The square root of 1221865721 is about 34955.1959084769. The cubic root of 1221865721 is about 1069.0741462617.

The spelling of 1221865721 in words is "one billion, two hundred twenty-one million, eight hundred sixty-five thousand, seven hundred twenty-one".