21515551 has 2 divisors, whose sum is σ = 21515552. Its totient is φ = 21515550.

The previous prime is 21515521. The next prime is 21515561. The reversal of 21515551 is 15551512.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21515551 - 2¹⁷ = 21384479 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21515521) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10757775 + 10757776.

It is an arithmetic number, because the mean of its divisors is an integer number (10757776).

Almost surely, 2^21515551 is an apocalyptic number.

21515551 is a deficient number, since it is larger than the sum of its proper divisors (1).

21515551 is an equidigital number, since it uses as much as digits as its factorization.

21515551 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1250, while the sum is 25.

The square root of 21515551 is about 4638.4858520858. The cubic root of 21515551 is about 278.1319138289.

The spelling of 21515551 in words is "twenty-one million, five hundred fifteen thousand, five hundred fifty-one".