20721605 has 8 divisors (see below), whose sum is σ = 25472664. Its totient is φ = 16172800.

The previous prime is 20721599. The next prime is 20721611. The reversal of 20721605 is 50612702.

It is an interprime number because it is at equal distance from previous prime (20721599) and next prime (20721611).

It can be written as a sum of positive squares in 4 ways, for example, as 2076481 + 18645124 = 1441^2 + 4318^2 .

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

It is not a de Polignac number, because 20721605 - 2⁶ = 20721541 is a prime.

It is a Duffinian number.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 50336 + ... + 50745.

It is an arithmetic number, because the mean of its divisors is an integer number (3184083).

Almost surely, 2^20721605 is an apocalyptic number.

It is an amenable number.

20721605 is a deficient number, since it is larger than the sum of its proper divisors (4751059).

20721605 is a wasteful number, since it uses less digits than its factorization.

20721605 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 101127.

The product of its (nonzero) digits is 840, while the sum is 23.

The square root of 20721605 is about 4552.0989664110. The cubic root of 20721605 is about 274.6678303422.

It can be divided in two parts, 207 and 21605, that added together give a palindrome (21812).

The spelling of 20721605 in words is "twenty million, seven hundred twenty-one thousand, six hundred five".