A357975 Divide all prime indices by 2, round down, and take the number with those prime indices, assuming prime(0) = 1.

1, 1, 2, 1, 2, 2, 3, 1, 4, 2, 3, 2, 5, 3, 4, 1, 5, 4, 7, 2, 6, 3, 7, 2, 4, 5, 8, 3, 11, 4, 11, 1, 6, 5, 6, 4, 13, 7, 10, 2, 13, 6, 17, 3, 8, 7, 17, 2, 9, 4, 10, 5, 19, 8, 6, 3, 14, 11, 19, 4, 23, 11, 12, 1, 10, 6, 23, 5, 14, 6, 29, 4, 29, 13, 8, 7, 9, 10, 31

Offset: 1

Author

Gus Wiseman, Oct 23 2022

Keywords

• nonn

Comments

A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798.

Also the Heinz number of the part-wise half (rounded down) of the partition with Heinz number n, where the Heinz number of a partition (y_1,...,y_k) is prime(y_1)*...*prime(y_k).

Each n appears A000005(n) times at odd positions (infinitely many at even). To see this, note that our transformation does not distinguish between A066207 and A066208.

Examples

			The prime indices of n = 1501500 are {1,1,2,3,3,3,4,5,6}, so the prime indices of a(n) are {1,1,1,1,2,2,3}; hence we have a(1501500) = 720.
The 6 odd positions of 2124 are: 63, 99, 105, 165, 175, 275, with prime indices:
   63: {2,2,4}
   99: {2,2,5}
  105: {2,3,4}
  165: {2,3,5}
  175: {3,3,4}
  275: {3,3,5}
		

Links

• SiYang Hu, Table of n, a(n) for n = 1..10000

Crossrefs

Positions of 1's are A000079.

Positions of 2's are 3 and A164095.

Positions of first appearances are A297002, sorted A066207.

A004526 is floor(n/2), with an extra first zero.

A056239 adds up prime indices, row-sums of A112798.

A109763 lists primes of index floor(n/2).

Cf. A003961, A064988, A064989, A076610, A215366, A248601, A357980.

Programs

Mathematica

Table[Times@@(If[#1<=2,1,Prime[Floor[PrimePi[#1]/2]]^#2]&@@@FactorInteger[n]),{n,100}]

Formula

Completely multiplicative with a(prime(2k)) = prime(k) and a(prime(2k+1)) = prime(k). Cf. A297002.

a(prime(n)) = A109763(n-1).