21681551 has 2 divisors, whose sum is σ = 21681552. Its totient is φ = 21681550.

The previous prime is 21681547. The next prime is 21681553. The reversal of 21681551 is 15518612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21681551 - 2² = 21681547 is a prime.

Together with 21681553, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21681553) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10840775 + 10840776.

It is an arithmetic number, because the mean of its divisors is an integer number (10840776).

Almost surely, 2^21681551 is an apocalyptic number.

21681551 is a deficient number, since it is larger than the sum of its proper divisors (1).

21681551 is an equidigital number, since it uses as much as digits as its factorization.

21681551 is an evil number, because the sum of its binary digits is even.

The product of its digits is 2400, while the sum is 29.

The square root of 21681551 is about 4656.3452406367. The cubic root of 21681551 is about 278.8453770827.

The spelling of 21681551 in words is "twenty-one million, six hundred eighty-one thousand, five hundred fifty-one".