640378021 has 2 divisors, whose sum is σ = 640378022. Its totient is φ = 640378020.

The previous prime is 640378009. The next prime is 640378073. The reversal of 640378021 is 120873046.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 568297921 + 72080100 = 23839^2 + 8490^2 .

It is a cyclic number.

It is not a de Polignac number, because 640378021 - 2¹⁵ = 640345253 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (640378001) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 320189010 + 320189011.

It is an arithmetic number, because the mean of its divisors is an integer number (320189011).

Almost surely, 2^640378021 is an apocalyptic number.

It is an amenable number.

640378021 is a deficient number, since it is larger than the sum of its proper divisors (1).

640378021 is an equidigital number, since it uses as much as digits as its factorization.

640378021 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 8064, while the sum is 31.

The square root of 640378021 is about 25305.6914744490. The cubic root of 640378021 is about 861.9435137753.

The spelling of 640378021 in words is "six hundred forty million, three hundred seventy-eight thousand, twenty-one".