21031781 has 2 divisors, whose sum is σ = 21031782. Its totient is φ = 21031780.

The previous prime is 21031753. The next prime is 21031793. The reversal of 21031781 is 18713012.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19096900 + 1934881 = 4370^2 + 1391^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21031781 is a prime.

It is a super-2 number, since 2×21031781² = 884671624063922, which contains 22 as substring.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21031721) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10515890 + 10515891.

It is an arithmetic number, because the mean of its divisors is an integer number (10515891).

Almost surely, 2^21031781 is an apocalyptic number.

It is an amenable number.

21031781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21031781 is an equidigital number, since it uses as much as digits as its factorization.

21031781 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 336, while the sum is 23.

The square root of 21031781 is about 4586.0419753857. The cubic root of 21031781 is about 276.0315242645.

Adding to 21031781 its reverse (18713012), we get a palindrome (39744793).

It can be divided in two parts, 21031 and 781, that added together give a palindrome (21812).

The spelling of 21031781 in words is "twenty-one million, thirty-one thousand, seven hundred eighty-one".