21261291 has 4 divisors (see below), whose sum is σ = 28348392. Its totient is φ = 14174192.

The previous prime is 21261287. The next prime is 21261293. The reversal of 21261291 is 19216212.

Added to its reverse (19216212) it gives a triangular number (40477503 = T₈₉₉₇).

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21261291 - 2² = 21261287 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21261293) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3543546 + ... + 3543551.

It is an arithmetic number, because the mean of its divisors is an integer number (7087098).

Almost surely, 2^21261291 is an apocalyptic number.

21261291 is a deficient number, since it is larger than the sum of its proper divisors (7087101).

21261291 is an equidigital number, since it uses as much as digits as its factorization.

21261291 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7087100.

The product of its digits is 432, while the sum is 24.

The square root of 21261291 is about 4610.9967469084. The cubic root of 21261291 is about 277.0319616271.

Adding to 21261291 its reverse (19216212), we get a triangular number (40477503 = T₈₉₉₇).

The spelling of 21261291 in words is "twenty-one million, two hundred sixty-one thousand, two hundred ninety-one".