A359527 - OEIS

A359527

Nonnegative numbers k such that if 2^i and 2^j appear in the binary expansion of k, then 2^(i OR j) also appears in the binary expansion of k (where OR denotes the bitwise OR operator).

0, 1, 2, 3, 4, 5, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 32, 33, 34, 35, 48, 49, 50, 51, 64, 65, 68, 69, 80, 81, 84, 85, 128, 129, 130, 131, 132, 133, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 152, 153, 160, 161, 162, 163, 164, 165, 168, 169, 170, 171

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OFFSET

1,3

COMMENTS

Equivalently, numbers whose binary expansions encode union-closed finite sets of finite sets of nonnegative integers:

- the encoding is based on a double application of A133457,

- for example: 11 -> {0, 1, 3} -> {{}, {0}, {0, 1}},

- a union-closed set f satisfies: for any i and j in f, the union of i and j belongs to f.

For any k >= 0, 2*k belongs to the sequence iff 2*k+1 belongs to the sequence.

This sequence has similarities with A190939; here we consider the bitwise OR operator, there the bitwise XOR operator.

This sequence is infinite as it contains the powers of 2.

LINKS

Table of n, a(n) for n=1..60.

Wikipedia, Union-closed sets conjecture

Index entries for sequences related to binary expansion of n

EXAMPLE

The first terms, alongside the corresponding union-closed sets, are:

n a(n) Union-closed set

---- ----- ----------------------

1 0 {}

2 1 {{}}

3 2 {{0}}

4 3 {{}, {0}}

5 4 {{1}}

6 5 {{}, {1}}

7 8 {{0, 1}}

8 9 {{}, {0, 1}}

9 10 {{0}, {0, 1}}

10 11 {{}, {0}, {0, 1}}

11 12 {{1}, {0, 1}}

12 13 {{}, {1}, {0, 1}}

13 14 {{0}, {1}, {0, 1}}

14 15 {{}, {0}, {1}, {0, 1}}

15 16 {{2}}

16 17 {{}, {2}}

17 32 {{0, 2}}

PROG

(PARI) is(n) = { my (b=vector(hammingweight(n))); for (i=1, #b, n -= 2^b[i] = valuation(n, 2)); setbinop(bitor, b)==b }

CROSSREFS

Cf. A133457, A190939 (XOR analog), A359528 (AND analog).

Sequence in context: A297163 A319024 A039260 * A039200 A039150 A008539

Adjacent sequences: A359524 A359525 A359526 * A359528 A359529 A359530

KEYWORD

nonn,base

AUTHOR

Rémy Sigrist, Jan 04 2023

STATUS

approved