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EXAMPLE
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a(0) = 0 because all positive integers can be written as a sum of 0th powers of primes, i.e. as sums of 1.
a(1) = 1 because 2^1 = 2, 3^1 = 3, hence all positive integers 2 or larger can be written as a*2 + b*3 for a,b nonnegative integers [2 = 2, 3 = 3, 4 = 2+2, 5 = 2+3, 6 = 2+2+2 = 3+3, 7 = 2+2+3,...].
a(2) = 23 because all integers 24 or larger can be written as a sum of squares and in fact as a sum of squares of primes.
a(3) = 154 because all integers 155 or larger can be written as a sum of cubes of primes.
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