20681721 has 6 divisors (see below), whose sum is σ = 29873610. Its totient is φ = 13787808.

The previous prime is 20681719. The next prime is 20681723. The reversal of 20681721 is 12718602.

It is an interprime number because it is at equal distance from previous prime (20681719) and next prime (20681723).

It can be written as a sum of positive squares in only one way, i.e., 15093225 + 5588496 = 3885^2 + 2364^2 .

It is not a de Polignac number, because 20681721 - 2¹ = 20681719 is a prime.

It is not an unprimeable number, because it can be changed into a prime (20681723) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1148976 + ... + 1148993.

It is an arithmetic number, because the mean of its divisors is an integer number (4978935).

Almost surely, 2^20681721 is an apocalyptic number.

It is an amenable number.

20681721 is a deficient number, since it is larger than the sum of its proper divisors (9191889).

20681721 is a wasteful number, since it uses less digits than its factorization.

20681721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2297975 (or 2297972 counting only the distinct ones).

The product of its (nonzero) digits is 1344, while the sum is 27.

The square root of 20681721 is about 4547.7160201578. The cubic root of 20681721 is about 274.4914944561.

The spelling of 20681721 in words is "twenty million, six hundred eighty-one thousand, seven hundred twenty-one".