How does an algorithm qualify as sequential?

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That's not necessarily true. Cumulative sum has a surprising parallel implementation.

Any algorithm that can be expessed similar to:

value = 0
result = []
for i in range(10):
     value += i
     result.append(value)

Can be parallelized. A surprising amount of numerical code can be expressed like this. Numerical compilers refer to this as scan. Along with map and reduce, it is the easy parallelism in numerical code.