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A350952
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The smallest number whose binary expansion has exactly n distinct runs.
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14
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0, 1, 2, 11, 38, 311, 2254, 36079, 549790, 17593311, 549687102, 35179974591, 2225029922430, 284803830071167, 36240869367020798, 9277662557957324543, 2368116566113212692990, 1212475681849964898811391, 619877748107024946567312382, 634754814061593545284927880191
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OFFSET
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0,3
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COMMENTS
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Positions of first appearances in A297770 (with offset 0).
The binary expansion of terms for n > 0 starts with 1, then floor(n/2) 0's, then alternates runs of increasing numbers of 1's, and decreasing numbers of 0's; see Python code. Thus, for n even, terms have n*(n/2+1)/2 binary digits, and for n odd, ((n+1) + (n-1)*((n-1)/2+1))/2 binary digits. - Michael S. Branicky, Feb 14 2022
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LINKS
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EXAMPLE
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The terms and their binary expansions begin:
0: ()
1: 1
2: 10
11: 1011
38: 100110
311: 100110111
2254: 100011001110
36079: 1000110011101111
549790: 10000110001110011110
For example, 311 has binary expansion 100110111 with 5 distinct runs: 1, 00, 11, 0, 111.
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MATHEMATICA
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q=Table[Length[Union[Split[If[n==0, {}, IntegerDigits[n, 2]]]]], {n, 0, 1000}]; Table[Position[q, i][[1, 1]]-1, {i, Union[q]}]
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PROG
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(Python)
def a(n): # returns term by construction
if n == 0: return 0
q, r = divmod(n, 2)
if r == 0:
s = "".join("1"*i + "0"*(q-i+1) for i in range(1, q+1))
assert len(s) == n*(n//2+1)//2
else:
s = "1" + "".join("0"*(q-i+2) + "1"*i for i in range(2, q+2))
assert len(s) == ((n+1) + (n-1)*((n-1)//2+1))//2
return int(s, 2)
(PARI) a(n)={my(t=0); for(k=1, (n+1)\2, t=((t<<k)+(2^k-1))<<(n\2+1-k)); t} \\ Andrew Howroyd, Feb 15 2022
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CROSSREFS
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Subset of A175413 (binary expansion has distinct runs), for lengths A044813.
The version for standard compositions is A351015.
A011782 counts integer compositions.
A242882 counts compositions with distinct multiplicities.
A318928 gives runs-resistance of binary expansion.
A334028 counts distinct parts in standard compositions.
A351014 counts distinct runs in standard compositions.
Counting words with all distinct runs:
- A351202 = permutations of prime factors.
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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