121905821 has 4 divisors (see below), whose sum is σ = 126109500. Its totient is φ = 117702144.

The previous prime is 121905803. The next prime is 121905829. The reversal of 121905821 is 128509121.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 75516100 + 46389721 = 8690^2 + 6811^2 .

It is a cyclic number.

It is not a de Polignac number, because 121905821 - 2¹⁸ = 121643677 is a prime.

It is a super-2 number, since 2×121905821² = 29722058387368082, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (29), and also a Moran number because the ratio is a prime number: 4203649 = 121905821 / (1 + 2 + 1 + 9 + 0 + 5 + 8 + 2 + 1).

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (121905829) 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, 2101796 + ... + 2101853.

It is an arithmetic number, because the mean of its divisors is an integer number (31527375).

Almost surely, 2^121905821 is an apocalyptic number.

It is an amenable number.

121905821 is a deficient number, since it is larger than the sum of its proper divisors (4203679).

121905821 is an equidigital number, since it uses as much as digits as its factorization.

121905821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4203678.

The product of its (nonzero) digits is 1440, while the sum is 29.

The square root of 121905821 is about 11041.0969110863. The cubic root of 121905821 is about 495.8399113189.

The spelling of 121905821 in words is "one hundred twenty-one million, nine hundred five thousand, eight hundred twenty-one".