18221561 has 2 divisors, whose sum is σ = 18221562. Its totient is φ = 18221560.

The previous prime is 18221549. The next prime is 18221563. The reversal of 18221561 is 16512281.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 11560000 + 6661561 = 3400^2 + 2581^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-18221561 is a prime.

Together with 18221563, it forms a pair of twin primes.

It is a Chen prime.

It is equal to p_1164509 and since 18221561 and 1164509 have the same sum of digits, it is a Honaker prime.

It is a self number, because there is not a number n which added to its sum of digits gives 18221561.

It is not a weakly prime, because it can be changed into another prime (18221563) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9110780 + 9110781.

It is an arithmetic number, because the mean of its divisors is an integer number (9110781).

Almost surely, 2^18221561 is an apocalyptic number.

It is an amenable number.

18221561 is a deficient number, since it is larger than the sum of its proper divisors (1).

18221561 is an equidigital number, since it uses as much as digits as its factorization.

18221561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 960, while the sum is 26.

The square root of 18221561 is about 4268.6720417479. The cubic root of 18221561 is about 263.1450428103.

It can be divided in two parts, 18221 and 561, that multiplied together give a triangular number (10221981 = T₄₅₂₁).

The spelling of 18221561 in words is "eighteen million, two hundred twenty-one thousand, five hundred sixty-one".