21129947 has 2 divisors, whose sum is σ = 21129948. Its totient is φ = 21129946.

The previous prime is 21129943. The next prime is 21129949. The reversal of 21129947 is 74992112.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21129947 - 2² = 21129943 is a prime.

Together with 21129949, it forms a pair of twin primes.

It is a Chen prime.

It is equal to p_1337678 and since 21129947 and 1337678 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 (21129943) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10564973 + 10564974.

It is an arithmetic number, because the mean of its divisors is an integer number (10564974).

Almost surely, 2^21129947 is an apocalyptic number.

21129947 is a deficient number, since it is larger than the sum of its proper divisors (1).

21129947 is an equidigital number, since it uses as much as digits as its factorization.

21129947 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 9072, while the sum is 35.

The square root of 21129947 is about 4596.7322088632. The cubic root of 21129947 is about 276.4603175789.

The spelling of 21129947 in words is "twenty-one million, one hundred twenty-nine thousand, nine hundred forty-seven".