21781157 has 2 divisors, whose sum is σ = 21781158. Its totient is φ = 21781156.

The previous prime is 21781139. The next prime is 21781171. The reversal of 21781157 is 75118712.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19061956 + 2719201 = 4366^2 + 1649^2 .

It is a cyclic number.

It is not a de Polignac number, because 21781157 - 2⁶ = 21781093 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21781187) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10890578 + 10890579.

It is an arithmetic number, because the mean of its divisors is an integer number (10890579).

Almost surely, 2^21781157 is an apocalyptic number.

It is an amenable number.

21781157 is a deficient number, since it is larger than the sum of its proper divisors (1).

21781157 is an equidigital number, since it uses as much as digits as its factorization.

21781157 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3920, while the sum is 32.

The square root of 21781157 is about 4667.0287121465. The cubic root of 21781157 is about 279.2717341603.

Adding to 21781157 its reverse (75118712), we get a palindrome (96899869).

The spelling of 21781157 in words is "twenty-one million, seven hundred eighty-one thousand, one hundred fifty-seven".