161661721 has 8 divisors (see below), whose sum is σ = 184338504. Its totient is φ = 140448000.

The previous prime is 161661707. The next prime is 161661739. The reversal of 161661721 is 127166161.

It can be written as a sum of positive squares in 4 ways, for example, as 2624400 + 159037321 = 1620^2 + 12611^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 161661721 - 2¹⁹ = 161137433 is a prime.

It is a super-2 number, since 2×161661721² = 52269024073363682, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (161661761) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 365530 + ... + 365971.

It is an arithmetic number, because the mean of its divisors is an integer number (23042313).

Almost surely, 2^161661721 is an apocalyptic number.

It is an amenable number.

161661721 is a deficient number, since it is larger than the sum of its proper divisors (22676783).

161661721 is a wasteful number, since it uses less digits than its factorization.

161661721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 731531.

The product of its digits is 3024, while the sum is 31.

The square root of 161661721 is about 12714.6262626945. The cubic root of 161661721 is about 544.7564728558.

The spelling of 161661721 in words is "one hundred sixty-one million, six hundred sixty-one thousand, seven hundred twenty-one".