21681421 has 2 divisors, whose sum is σ = 21681422. Its totient is φ = 21681420.

The previous prime is 21681389. The next prime is 21681449. The reversal of 21681421 is 12418612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18922500 + 2758921 = 4350^2 + 1661^2 .

It is a cyclic number.

It is not a de Polignac number, because 21681421 - 2⁵ = 21681389 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21681481) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10840710 + 10840711.

It is an arithmetic number, because the mean of its divisors is an integer number (10840711).

Almost surely, 2^21681421 is an apocalyptic number.

It is an amenable number.

21681421 is a deficient number, since it is larger than the sum of its proper divisors (1).

21681421 is an equidigital number, since it uses as much as digits as its factorization.

21681421 is an evil number, because the sum of its binary digits is even.

The product of its digits is 768, while the sum is 25.

The square root of 21681421 is about 4656.3312811698. The cubic root of 21681421 is about 278.8448197737.

The spelling of 21681421 in words is "twenty-one million, six hundred eighty-one thousand, four hundred twenty-one".