21003781 has 2 divisors, whose sum is σ = 21003782. Its totient is φ = 21003780.

The previous prime is 21003751. The next prime is 21003803. The reversal of 21003781 is 18730012.

It is an a-pointer prime, because the next prime (21003803) can be obtained adding 21003781 to its sum of digits (22).

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16273156 + 4730625 = 4034^2 + 2175^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21003781 is a prime.

It is a super-2 number, since 2×21003781² = 882317632591922, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21003751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10501890 + 10501891.

It is an arithmetic number, because the mean of its divisors is an integer number (10501891).

Almost surely, 2^21003781 is an apocalyptic number.

It is an amenable number.

21003781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21003781 is an equidigital number, since it uses as much as digits as its factorization.

21003781 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 22.

The square root of 21003781 is about 4582.9882173097. The cubic root of 21003781 is about 275.9089745688.

Adding to 21003781 its reverse (18730012), we get a palindrome (39733793).

The spelling of 21003781 in words is "twenty-one million, three thousand, seven hundred eighty-one".