A372851 Squarefree numbers whose prime indices are the binary indices of some prime number.

3, 6, 10, 22, 30, 42, 46, 66, 70, 102, 114, 118, 130, 182, 238, 246, 266, 318, 330, 354, 370, 402, 406, 434, 442, 510, 546, 646, 654, 690, 762, 770, 798, 930, 938, 946, 962, 986, 1066, 1102, 1122, 1178, 1218, 1222, 1246, 1258, 1334, 1378, 1430, 1482, 1578

Offset

1,1

Comments

A binary index of n is any position of a 1 in its reversed binary expansion. The binary indices of n are row n of A048793.

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.

Note the function taking a set s to its rank Sum_i 2^(s_i-1) is the inverse of A048793 (binary indices).

Links

Table of n, a(n) for n=1..51.

Formula

Squarefree numbers k such that Sum_{i:prime(i)|k} 2^(i-1) is prime, where the sum is over the (distinct) prime indices of k.

Example

The prime indices of 70 are {1,3,4}, which are the binary indices of 13, which is prime, so 70 is in the sequence.
The prime indices of 15 are {2,3}, which are the binary indices of 6, which is not prime, so 15 is not in the sequence.
The terms together with their prime indices begin:
    3: {2}
    6: {1,2}
   10: {1,3}
   22: {1,5}
   30: {1,2,3}
   42: {1,2,4}
   46: {1,9}
   66: {1,2,5}
   70: {1,3,4}
  102: {1,2,7}
  114: {1,2,8}
  118: {1,17}
  130: {1,3,6}
  182: {1,4,6}
  238: {1,4,7}
  246: {1,2,13}
  266: {1,4,8}
  318: {1,2,16}
  330: {1,2,3,5}
  354: {1,2,17}
  370: {1,3,12}
  402: {1,2,19}

Mathematica

Select[Range[100], SquareFreeQ[#] && PrimeQ[Total[2^(PrimePi/@First/@FactorInteger[#]-1)]]&]

Crossrefs

[Warning: do not confuse A372887 with the strict case A372687.]

For odd instead of prime we have A039956.

For even instead of prime we have A056911.

Strict partitions of this type are counted by A372687.

Non-strict partitions of this type are counted by A372688, ranks A277319.

The nonsquarefree version is A372850, counted by A372887.

A014499 lists binary indices of prime numbers.

A019565 gives Heinz number of binary indices, adjoint A048675.

A038499 counts partitions of prime length, strict A085756.

A048793 and A272020 (reverse) list binary indices:

- length A000120

- min A001511

- sum A029931

- max A070939

A058698 counts partitions of prime numbers, strict A064688.

A372885 lists primes whose binary indices sum to a prime, indices A372886.

Cf. A000040, A005940, A025147, A035100, A071814, A096111, A096765, A231204, A372429, A372471, A372689.

Sequence in context: A346733 A220827 A063015 * A362909 A282515 A284202

Adjacent sequences: A372848 A372849 A372850 * A372852 A372853 A372854

Keyword

nonn

Author

Gus Wiseman, May 16 2024

Status

approved