161647721 has 4 divisors (see below), whose sum is σ = 164697732. Its totient is φ = 158597712.

The previous prime is 161647699. The next prime is 161647771. The reversal of 161647721 is 127746161.

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: 127746161 = 113 ⋅1130497.

It can be written as a sum of positive squares in 2 ways, for example, as 134815321 + 26832400 = 11611^2 + 5180^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-161647721 is a prime.

It is a super-2 number, since 2×161647721² = 52259971408987682, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (161647771) 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, 1524926 + ... + 1525031.

It is an arithmetic number, because the mean of its divisors is an integer number (41174433).

Almost surely, 2^161647721 is an apocalyptic number.

It is an amenable number.

161647721 is a deficient number, since it is larger than the sum of its proper divisors (3050011).

161647721 is an equidigital number, since it uses as much as digits as its factorization.

161647721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3050010.

The product of its digits is 14112, while the sum is 35.

The square root of 161647721 is about 12714.0757037230. The cubic root of 161647721 is about 544.7407469917.

The spelling of 161647721 in words is "one hundred sixty-one million, six hundred forty-seven thousand, seven hundred twenty-one".