21478781 has 2 divisors, whose sum is σ = 21478782. Its totient is φ = 21478780.

The previous prime is 21478741. The next prime is 21478783. The reversal of 21478781 is 18787412.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12538681 + 8940100 = 3541^2 + 2990^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21478781 is a prime.

It is a super-2 number, since 2×21478781² = 922676066491922, which contains 22 as substring.

Together with 21478783, it forms a pair of twin primes.

It is a Chen prime.

It is an alternating number because its digits alternate between even and odd.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21478783) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10739390 + 10739391.

It is an arithmetic number, because the mean of its divisors is an integer number (10739391).

Almost surely, 2^21478781 is an apocalyptic number.

It is an amenable number.

21478781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21478781 is an equidigital number, since it uses as much as digits as its factorization.

21478781 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 25088, while the sum is 38.

The square root of 21478781 is about 4634.5205793048. The cubic root of 21478781 is about 277.9733813342.

The spelling of 21478781 in words is "twenty-one million, four hundred seventy-eight thousand, seven hundred eighty-one".