20640643 has 4 divisors (see below), whose sum is σ = 20777488. Its totient is φ = 20503800.

The previous prime is 20640617. The next prime is 20640647. The reversal of 20640643 is 34604602.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 34604602 = 2 ⋅17302301.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-20640643 is a prime.

It is a super-2 number, since 2×20640643² = 852072286906898, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20640647) 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, 68196 + ... + 68497.

It is an arithmetic number, because the mean of its divisors is an integer number (5194372).

Almost surely, 2^20640643 is an apocalyptic number.

20640643 is a deficient number, since it is larger than the sum of its proper divisors (136845).

20640643 is a wasteful number, since it uses less digits than its factorization.

20640643 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 136844.

The product of its (nonzero) digits is 3456, while the sum is 25.

The square root of 20640643 is about 4543.1974423307. The cubic root of 20640643 is about 274.3096424877.

The spelling of 20640643 in words is "twenty million, six hundred forty thousand, six hundred forty-three".