161821551 has 4 divisors (see below), whose sum is σ = 215762072. Its totient is φ = 107881032.

The previous prime is 161821507. The next prime is 161821573. The reversal of 161821551 is 155128161.

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

It is not a de Polignac number, because 161821551 - 2⁹ = 161821039 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (161821501) 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, 26970256 + ... + 26970261.

It is an arithmetic number, because the mean of its divisors is an integer number (53940518).

Almost surely, 2^161821551 is an apocalyptic number.

161821551 is a deficient number, since it is larger than the sum of its proper divisors (53940521).

161821551 is an equidigital number, since it uses as much as digits as its factorization.

161821551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 53940520.

The product of its digits is 2400, while the sum is 30.

The square root of 161821551 is about 12720.9099910344. The cubic root of 161821551 is about 544.9359417458.

The spelling of 161821551 in words is "one hundred sixty-one million, eight hundred twenty-one thousand, five hundred fifty-one".