Complex sentence: For the most typical case, a string of bits, this is the number of 1's in the string, or the digit sum of the binary representation of a given number and the "ℓ" ₁ norm of a bit vector.
Split into simpler sentences: For the most typical case, a string of bits, this is the number of 1's in the string. It is the digit sum of the binary representation of a given number and the "ℓ" ₁ norm of a bit vector.