21324781 has 2 divisors, whose sum is σ = 21324782. Its totient is φ = 21324780.

The previous prime is 21324769. The next prime is 21324791. The reversal of 21324781 is 18742312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18404100 + 2920681 = 4290^2 + 1709^2 .

It is a cyclic number.

It is not a de Polignac number, because 21324781 - 2¹⁵ = 21292013 is a prime.

It is a super-2 number, since 2×21324781² = 909492569395922, which contains 22 as substring.

It is equal to p_1349209 and since 21324781 and 1349209 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21324701) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10662390 + 10662391.

It is an arithmetic number, because the mean of its divisors is an integer number (10662391).

Almost surely, 2^21324781 is an apocalyptic number.

It is an amenable number.

21324781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21324781 is an equidigital number, since it uses as much as digits as its factorization.

21324781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 2688, while the sum is 28.

The square root of 21324781 is about 4617.8762434695. The cubic root of 21324781 is about 277.3074431820.

The spelling of 21324781 in words is "twenty-one million, three hundred twenty-four thousand, seven hundred eighty-one".