15691721 has 4 divisors (see below), whose sum is σ = 15700944. Its totient is φ = 15682500.

The previous prime is 15691703. The next prime is 15691729. The reversal of 15691721 is 12719651.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 12719651 = 7 ⋅1817093.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-15691721 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (15691729) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1235 + ... + 5736.

It is an arithmetic number, because the mean of its divisors is an integer number (3925236).

Almost surely, 2^15691721 is an apocalyptic number.

It is an amenable number.

15691721 is a deficient number, since it is larger than the sum of its proper divisors (9223).

15691721 is an equidigital number, since it uses as much as digits as its factorization.

15691721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9222.

The product of its digits is 3780, while the sum is 32.

The square root of 15691721 is about 3961.2776979152. The cubic root of 15691721 is about 250.3553400279.

The spelling of 15691721 in words is "fifteen million, six hundred ninety-one thousand, seven hundred twenty-one".