21689947 has 2 divisors, whose sum is σ = 21689948. Its totient is φ = 21689946.

The previous prime is 21689917. The next prime is 21689959. The reversal of 21689947 is 74998612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21689947 - 2¹⁵ = 21657179 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21689897 and 21689906.

It is not a weakly prime, because it can be changed into another prime (21689917) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10844973 + 10844974.

It is an arithmetic number, because the mean of its divisors is an integer number (10844974).

Almost surely, 2^21689947 is an apocalyptic number.

21689947 is a deficient number, since it is larger than the sum of its proper divisors (1).

21689947 is an equidigital number, since it uses as much as digits as its factorization.

21689947 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 217728, while the sum is 46.

The square root of 21689947 is about 4657.2467188243. The cubic root of 21689947 is about 278.8813659541.

The spelling of 21689947 in words is "twenty-one million, six hundred eighty-nine thousand, nine hundred forty-seven".