21781261 has 2 divisors, whose sum is σ = 21781262. Its totient is φ = 21781260.

The previous prime is 21781259. The next prime is 21781273. The reversal of 21781261 is 16218712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 14969161 + 6812100 = 3869^2 + 2610^2 .

It is a cyclic number.

It is not a de Polignac number, because 21781261 - 2¹ = 21781259 is a prime.

Together with 21781259, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21781211) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10890630 + 10890631.

It is an arithmetic number, because the mean of its divisors is an integer number (10890631).

Almost surely, 2^21781261 is an apocalyptic number.

It is an amenable number.

21781261 is a deficient number, since it is larger than the sum of its proper divisors (1).

21781261 is an equidigital number, since it uses as much as digits as its factorization.

21781261 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1344, while the sum is 28.

The square root of 21781261 is about 4667.0398541260. The cubic root of 21781261 is about 279.2721786456.

Adding to 21781261 its reverse (16218712), we get a palindrome (37999973).

The spelling of 21781261 in words is "twenty-one million, seven hundred eighty-one thousand, two hundred sixty-one".