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A266511
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Minimal difference between the smallest and largest of n consecutive large primes that form a symmetric n-tuplet as permitted by divisibility considerations.
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9
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0, 2, 12, 8, 36, 16, 60, 26, 84, 34, 132, 46, 168, 56, 180, 74, 240, 82, 252, 94, 324, 106, 372, 118, 420, 134, 432, 142, 492, 146, 540, 158, 600, 166, 648, 178, 660, 194, 720, 202, 780, 214, 816, 226, 840, 254, 912, 262, 1020, 278
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OFFSET
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1,2
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COMMENTS
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For the definition of n-tuplet and minimal differences without the symmetry restriction, see A008407. In particular, a(n) >= A008407(n).
An n-tuplet (p(1),...,p(n)) is symmetric if p(k) + p(n+1-k) is the same for all k=1,2,...,n (cf. A175309).
Smallest primes starting a shortest symmetric n-tuplet are given in A266512.
For odd n, a(n) is divisible by 12.
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LINKS
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EXAMPLE
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For n=3, any shortest symmetric n-tuplet has the form (p, p+6, p+12) and thus a(3)=12.
For each n-tuplet (p(1), ..., p(n)) with odd n, let m be its middle prime, i.e., m = p((n+1)/2). Then, since (by symmetry) (p(k) + p(n+1-k))/2 = m for all k = 1..n, we can define the n-tuplet by m and its vector of differences d(j) = m - p(j) for j = 1..(n-1)/2. In other words, given m and d(j) for j = 1..(n-1)/2, the (n-1)/2 primes below m are given by p(j) = m - d(j), and the (n-1)/2 primes above m are given by p(n+1-j) = m + d(j); the difference p(n) - p(1) is thus (m + d(1)) - (m - d(1)) = 2*d(1).
For example, one symmetric 7-tuplet of consecutive primes is (12003179, 12003191, 12003197, 12003209, 12003221, 12003227, 12003239), which can be written as (m-30, m-18, m-12, m, m+12, m+18, m+30) where m=12003209; here we have d(1)=30, d(2)=18, d(3)=12. Among all symmetric 7-tuplets of consecutive primes that satisfy divisibility considerations, the minimal value of d(1) is, in fact, 30, so a(7) = 2*30 = 60.
For n = 3, 5, ..., 29, the lexicographically first vector (d(1), d(2), ..., d((n-1)/2)) permitted by divisibility considerations is as follows:
n| 1 2 3 4 5 6 7 8 9 10 11 12 13 14
--+-------------------------------------------------------
3| 6
5| 18 12
7| 30 18 12
9| 42 30 18 12
11| 66 60 36 24 6
13| 84 66 60 36 24 6
15| 90 84 66 60 36 24 6
17|120 108 90 78 60 48 42 18
19|126 120 114 96 84 54 36 30 6
21|162 150 132 120 108 102 78 48 42 18
23|186 180 150 144 126 96 84 66 60 54 30
25|210 186 180 150 144 126 96 84 66 60 54 30
27|216 210 204 180 126 120 114 96 84 54 36 30 6
29|246 216 210 204 186 174 144 126 90 84 66 60 24 6
(End)
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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a(15) and a(17)-a(18) from Jaroslaw Wroblewski
a(19), a(21), a(23), a(25), a(27), a(29) from Jon E. Schoenfield, Jan 02 2016, Jan 05 2016
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STATUS
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approved
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