21787 has 2 divisors, whose sum is σ = 21788. Its totient is φ = 21786.

The previous prime is 21773. The next prime is 21799. The reversal of 21787 is 78712.

It is a happy number.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21787 - 2¹¹ = 19739 is a prime.

21787 is a lucky number.

It is a plaindrome in base 13.

It is not a weakly prime, because it can be changed into another prime (21727) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10893 + 10894.

It is an arithmetic number, because the mean of its divisors is an integer number (10894).

2²¹⁷⁸⁷ is an apocalyptic number.

21787 is a deficient number, since it is larger than the sum of its proper divisors (1).

21787 is an equidigital number, since it uses as much as digits as its factorization.

21787 is an evil number, because the sum of its binary digits is even.

The product of its digits is 784, while the sum is 25.

The square root of 21787 is about 147.6042004822. The cubic root of 21787 is about 27.9296704350.

Adding to 21787 its sum of digits (25), we get a palindrome (21812).

It can be divided in two parts, 21 and 787, that added together give a palindrome (808).

The spelling of 21787 in words is "twenty-one thousand, seven hundred eighty-seven".