20721761 has 12 divisors (see below), whose sum is σ = 22251924. Its totient is φ = 19288800.

The previous prime is 20721749. The next prime is 20721763. The reversal of 20721761 is 16712702.

It can be written as a sum of positive squares in 2 ways, for example, as 5550736 + 15171025 = 2356^2 + 3895^2 .

It is not a de Polignac number, because 20721761 - 2²² = 16527457 is a prime.

It is a super-2 number, since 2×20721761² = 858782757882242, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 20721761.

It is not an unprimeable number, because it can be changed into a prime (20721763) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 21551 + ... + 22491.

It is an arithmetic number, because the mean of its divisors is an integer number (1854327).

Almost surely, 2^20721761 is an apocalyptic number.

It is an amenable number.

20721761 is a deficient number, since it is larger than the sum of its proper divisors (1530163).

20721761 is an equidigital number, since it uses as much as digits as its factorization.

20721761 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1040 (or 1021 counting only the distinct ones).

The product of its (nonzero) digits is 1176, while the sum is 26.

The square root of 20721761 is about 4552.1161013313. The cubic root of 20721761 is about 274.6685196079.

The spelling of 20721761 in words is "twenty million, seven hundred twenty-one thousand, seven hundred sixty-one".