A378123 a(n) = number of prime divisors of the sum of the first n odd primes.
1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 3, 2, 2, 2, 1, 3, 1, 2, 2, 3, 2, 2, 1, 2, 2, 2, 1, 2, 2, 2, 1, 3, 2, 3, 2, 2, 3, 3, 2, 3, 3, 3, 2, 4, 2, 2, 2, 3, 4, 3, 2, 3, 1, 3, 1, 3, 2, 3, 2, 5, 1, 5, 2, 4, 1, 3, 2, 3, 3, 3, 1, 3, 2, 3, 1, 3, 2, 3, 2, 4, 3, 3, 2, 2, 2, 4
Offset: 1
Keywords
Examples
3+5+7+11+13 = 39 = 3*13, so a(5) = 2.
Programs
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Mathematica
a[n_] := PrimeNu[Total[Prime[1+Range[n]]]]; Array[a, 500]
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PARI
a378123(n)=omega(sum(k=2,n+1,prime(k))) \\ Hugo Pfoertner, Nov 19 2024