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A308299
Numbers whose prime indices are factorial numbers.
2
1, 2, 3, 4, 6, 8, 9, 12, 13, 16, 18, 24, 26, 27, 32, 36, 39, 48, 52, 54, 64, 72, 78, 81, 89, 96, 104, 108, 117, 128, 144, 156, 162, 169, 178, 192, 208, 216, 234, 243, 256, 267, 288, 312, 324, 338, 351, 356, 384, 416, 432, 468, 486, 507, 512, 534, 576, 624, 648
OFFSET
1,2
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. The Heinz number of an integer partition (y_1,...,y_k) is prime(y_1)*...*prime(y_k), so these are Heinz numbers of integer partitions using factorial numbers. The enumeration of these partitions by sum is given by A064986.
LINKS
FORMULA
Sum_{n>=1} 1/a(n) = 1/Product_{k>=1} (1 - 1/prime(k!)) = 3.292606708493... . - Amiram Eldar, Dec 03 2022
EXAMPLE
The sequence of terms together with their prime indices begins:
1: {}
2: {1}
3: {2}
4: {1,1}
6: {1,2}
8: {1,1,1}
9: {2,2}
12: {1,1,2}
13: {6}
16: {1,1,1,1}
18: {1,2,2}
24: {1,1,1,2}
26: {1,6}
27: {2,2,2}
32: {1,1,1,1,1}
36: {1,1,2,2}
39: {2,6}
48: {1,1,1,1,2}
52: {1,1,6}
54: {1,2,2,2}
MATHEMATICA
nn=5;
facts=Array[Factorial, nn];
Select[Range[Prime[Max@@facts]], SubsetQ[facts, PrimePi/@First/@FactorInteger[#]]&]
KEYWORD
nonn
AUTHOR
Gus Wiseman, May 19 2019
STATUS
approved