21328781 has 2 divisors, whose sum is σ = 21328782. Its totient is φ = 21328780.

The previous prime is 21328763. The next prime is 21328787. The reversal of 21328781 is 18782312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16080100 + 5248681 = 4010^2 + 2291^2 .

It is a cyclic number.

It is not a de Polignac number, because 21328781 - 2²² = 17134477 is a prime.

It is a super-2 number, since 2×21328781² = 909833797891922, which contains 22 as substring.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21328787) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10664390 + 10664391.

It is an arithmetic number, because the mean of its divisors is an integer number (10664391).

Almost surely, 2^21328781 is an apocalyptic number.

It is an amenable number.

21328781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21328781 is an equidigital number, since it uses as much as digits as its factorization.

21328781 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 5376, while the sum is 32.

The square root of 21328781 is about 4618.3093226851. The cubic root of 21328781 is about 277.3247807641.

It can be divided in two parts, 2 and 1328781, that multiplied together give a palindrome (2657562).

The spelling of 21328781 in words is "twenty-one million, three hundred twenty-eight thousand, seven hundred eighty-one".