21291217 has 2 divisors, whose sum is σ = 21291218. Its totient is φ = 21291216.

The previous prime is 21291197. The next prime is 21291239. The reversal of 21291217 is 71219212.

21291217 is an esthetic number in base 7, because in such base its adjacent digits differ by 1.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20034576 + 1256641 = 4476^2 + 1121^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21291217 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21291191 and 21291200.

It is not a weakly prime, because it can be changed into another prime (21291247) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10645608 + 10645609.

It is an arithmetic number, because the mean of its divisors is an integer number (10645609).

Almost surely, 2^21291217 is an apocalyptic number.

It is an amenable number.

21291217 is a deficient number, since it is larger than the sum of its proper divisors (1).

21291217 is an equidigital number, since it uses as much as digits as its factorization.

21291217 is an evil number, because the sum of its binary digits is even.

The product of its digits is 504, while the sum is 25.

The square root of 21291217 is about 4614.2406742605. The cubic root of 21291217 is about 277.1618780375.

The spelling of 21291217 in words is "twenty-one million, two hundred ninety-one thousand, two hundred seventeen".