21356381 has 2 divisors, whose sum is σ = 21356382. Its totient is φ = 21356380.

The previous prime is 21356353. The next prime is 21356429. The reversal of 21356381 is 18365312.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18567481 + 2788900 = 4309^2 + 1670^2 .

It is a cyclic number.

It is not a de Polignac number, because 21356381 - 2²² = 17162077 is a prime.

It is a super-2 number, since 2×21356381² = 912190018834322, which contains 22 as substring.

It is a pancake number, because a pancake can be divided into 21356381 parts by 6535 straight cuts.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21356341) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10678190 + 10678191.

It is an arithmetic number, because the mean of its divisors is an integer number (10678191).

Almost surely, 2^21356381 is an apocalyptic number.

It is an amenable number.

21356381 is a deficient number, since it is larger than the sum of its proper divisors (1).

21356381 is an equidigital number, since it uses as much as digits as its factorization.

21356381 is an evil number, because the sum of its binary digits is even.

The product of its digits is 4320, while the sum is 29.

The square root of 21356381 is about 4621.2964631151. The cubic root of 21356381 is about 277.4443510411. Note that the first 3 decimals are identical.

The spelling of 21356381 in words is "twenty-one million, three hundred fifty-six thousand, three hundred eighty-one".