1216921821 has 4 divisors (see below), whose sum is σ = 1622562432. Its totient is φ = 811281212.

The previous prime is 1216921799. The next prime is 1216921843. The reversal of 1216921821 is 1281296121.

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.

It is an interprime number because it is at equal distance from previous prime (1216921799) and next prime (1216921843).

It is a cyclic number.

It is not a de Polignac number, because 1216921821 - 2⁹ = 1216921309 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1216921891) 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, 202820301 + ... + 202820306.

It is an arithmetic number, because the mean of its divisors is an integer number (405640608).

Almost surely, 2^1216921821 is an apocalyptic number.

It is an amenable number.

1216921821 is a deficient number, since it is larger than the sum of its proper divisors (405640611).

1216921821 is an equidigital number, since it uses as much as digits as its factorization.

1216921821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 405640610.

The product of its digits is 3456, while the sum is 33.

The square root of 1216921821 is about 34884.4065593784. The cubic root of 1216921821 is about 1067.6303050336.

The spelling of 1216921821 in words is "one billion, two hundred sixteen million, nine hundred twenty-one thousand, eight hundred twenty-one".