21517621 has 2 divisors, whose sum is σ = 21517622. Its totient is φ = 21517620.

The previous prime is 21517591. The next prime is 21517649. The reversal of 21517621 is 12671512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 18610596 + 2907025 = 4314^2 + 1705^2 .

It is a cyclic number.

It is not a de Polignac number, because 21517621 - 2⁹ = 21517109 is a prime.

It is equal to p_1360618 and since 21517621 and 1360618 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 (21517021) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10758810 + 10758811.

It is an arithmetic number, because the mean of its divisors is an integer number (10758811).

Almost surely, 2^21517621 is an apocalyptic number.

It is an amenable number.

21517621 is a deficient number, since it is larger than the sum of its proper divisors (1).

21517621 is an equidigital number, since it uses as much as digits as its factorization.

21517621 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 840, while the sum is 25.

The square root of 21517621 is about 4638.7089798779. The cubic root of 21517621 is about 278.1408331852.

The spelling of 21517621 in words is "twenty-one million, five hundred seventeen thousand, six hundred twenty-one".