OFFSET
0,3
COMMENTS
A primitive set of integers has no pair of elements one of which divides the other. Each element i in a subset contributes 2^(i-1) to the binary value for that subset. The integers missing from the sequence correspond to nonprimitive subsets.
REFERENCES
Alan Sutcliffe, Divisors and Common Factors in Sets of Integers, awaiting publication
EXAMPLE
a(10)=22 since the 10th primitive set counting from 0 is {5,3,2}, which maps onto 10110 binary = 22 decimal.
From Gus Wiseman, Oct 31 2019: (Start)
The sequence of terms together with their binary expansions and binary indices begins:
0: 0 ~ {}
1: 1 ~ {1}
2: 10 ~ {2}
4: 100 ~ {3}
6: 110 ~ {2,3}
8: 1000 ~ {4}
12: 1100 ~ {3,4}
16: 10000 ~ {5}
18: 10010 ~ {2,5}
20: 10100 ~ {3,5}
22: 10110 ~ {2,3,5}
24: 11000 ~ {4,5}
28: 11100 ~ {3,4,5}
(End)
MATHEMATICA
stableQ[u_, Q_]:=!Apply[Or, Outer[#1=!=#2&&Q[#1, #2]&, u, u, 1], {0, 1}];
Select[Range[0, 100], stableQ[Join@@Position[Reverse[IntegerDigits[#, 2]], 1], Divisible]&] (* Gus Wiseman, Oct 31 2019 *)
CROSSREFS
A051026 gives the number of primitive subsets of the integers 1 to n.
The version for prime indices (rather than binary indices) is A316476.
The relatively prime case is A328671.
Partitions with no consecutive divisible parts are A328171.
Compositions without consecutive divisible parts are A328460.
A ranking of antichains is A326704.
KEYWORD
easy,nonn,base
AUTHOR
Alan Sutcliffe (alansut(AT)ntlworld.com), Aug 14 2003
STATUS
approved