21555491 has 2 divisors, whose sum is σ = 21555492. Its totient is φ = 21555490.

The previous prime is 21555467. The next prime is 21555493. The reversal of 21555491 is 19455512.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21555491 - 2¹⁴ = 21539107 is a prime.

Together with 21555493, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21555493) 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, 10777745 + 10777746.

It is an arithmetic number, because the mean of its divisors is an integer number (10777746).

Almost surely, 2^21555491 is an apocalyptic number.

21555491 is a deficient number, since it is larger than the sum of its proper divisors (1).

21555491 is an equidigital number, since it uses as much as digits as its factorization.

21555491 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 9000, while the sum is 32.

The square root of 21555491 is about 4642.7891401613. The cubic root of 21555491 is about 278.3039091448.

The spelling of 21555491 in words is "twenty-one million, five hundred fifty-five thousand, four hundred ninety-one".