21278021 has 2 divisors, whose sum is σ = 21278022. Its totient is φ = 21278020.

The previous prime is 21277987. The next prime is 21278041. The reversal of 21278021 is 12087212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21252100 + 25921 = 4610^2 + 161^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21278021 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is equal to p_1346423 and since 21278021 and 1346423 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21278041) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10639010 + 10639011.

It is an arithmetic number, because the mean of its divisors is an integer number (10639011).

Almost surely, 2^21278021 is an apocalyptic number.

It is an amenable number.

21278021 is a deficient number, since it is larger than the sum of its proper divisors (1).

21278021 is an equidigital number, since it uses as much as digits as its factorization.

21278021 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 448, while the sum is 23.

The square root of 21278021 is about 4612.8105315523. The cubic root of 21278021 is about 277.1046058454.

The spelling of 21278021 in words is "twenty-one million, two hundred seventy-eight thousand, twenty-one".