21640747 has 2 divisors, whose sum is σ = 21640748. Its totient is φ = 21640746.

The previous prime is 21640721. The next prime is 21640763. The reversal of 21640747 is 74704612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21640747 - 2¹⁵ = 21607979 is a prime.

It is a super-3 number, since 3×21640747³ (a number of 23 digits) contains 333 as substring.

It is equal to p_1367923 and since 21640747 and 1367923 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (21640147) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10820373 + 10820374.

It is an arithmetic number, because the mean of its divisors is an integer number (10820374).

Almost surely, 2^21640747 is an apocalyptic number.

21640747 is a deficient number, since it is larger than the sum of its proper divisors (1).

21640747 is an equidigital number, since it uses as much as digits as its factorization.

21640747 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 9408, while the sum is 31.

The square root of 21640747 is about 4651.9616292485. The cubic root of 21640747 is about 278.6703411436.

The spelling of 21640747 in words is "twenty-one million, six hundred forty thousand, seven hundred forty-seven".