161987821 has 4 divisors (see below), whose sum is σ = 162227772. Its totient is φ = 161747872.

The previous prime is 161987809. The next prime is 161987831. The reversal of 161987821 is 128789161.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 128789161 = 17 ⋅7575833.

It can be written as a sum of positive squares in 2 ways, for example, as 133749225 + 28238596 = 11565^2 + 5314^2 .

It is a cyclic number.

It is not a de Polignac number, because 161987821 - 2²¹ = 159890669 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 (161987801) 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, 118960 + ... + 120313.

It is an arithmetic number, because the mean of its divisors is an integer number (40556943).

Almost surely, 2^161987821 is an apocalyptic number.

It is an amenable number.

161987821 is a deficient number, since it is larger than the sum of its proper divisors (239951).

161987821 is an equidigital number, since it uses as much as digits as its factorization.

161987821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 239950.

The product of its digits is 48384, while the sum is 43.

The square root of 161987821 is about 12727.4436160605. The cubic root of 161987821 is about 545.1225165741.

The spelling of 161987821 in words is "one hundred sixty-one million, nine hundred eighty-seven thousand, eight hundred twenty-one".