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A '''sparse ruler''' is a ruler in which some of the distance marks may be missing.  More abstractly, a sparse ruler of length <math>L</math> with <math>m</math> marks is a sequence of integers <math>a_1, a_2, ..., a_m</math> where <math>0 = a_1 < a_2 < ... < a_m = L</math>.  The marks <math>a_1</math> and <math>a_m</math> correspond to the ends of the ruler.  In order to measure the distance <math>K</math>, with <math>0<=K<=L</math> there must be marks <math>a_i</math> and <math>a_j</math> such that <math>a_j-a_i=K</math>.

A ''complete'' sparse ruler allows you to measure any integer distance up to its full length.  A complete sparse ruler is called ''minimal'' if there is no complete sparse ruler of length <math>L</math> with <math>m-1</math> marks.  In other words, if any of the marks is removed one can no longer measure all of the distances, even if the marks could be rearranged.  A complete sparse ruler is called ''maximal'' if there is no complete sparse ruler of length <math>L+1</math> with <math>m</math> marks.  A sparse ruler is called ''optimal'' if it is both minimal and maximal.

Since the number of distinct pairs of marks is <math>m(m-1)/2</math>, this is an upper bound on the length <math>L</math> of any maximal sparse ruler with <math>m</math> marks.  This upper bound can be achieved only for 2, 3 or 4 marks.  For larger numbers of marks, the difference between the optimal length and the bound grows gradually, and unevenly.

For example, for 6 marks the upper bound is 15, but the maximal length is 13.  There are 3 different configurations of sparse rulers of length 13 with 6 marks.  One is {0, 1, 2, 6, 10, 13}.  To measure a length of 7, say, with this ruler you would take the distance between the marks at 6 and 13.

A [[Golomb ruler]] is a sparse ruler that requires all of the differences <math>a_j-a_i</math> be distinct.  In general, a Golomb ruler with <math>m</math> marks will be considerably longer than an optimal sparse ruler with <math>m</math> marks, since <math>m(m-1)/2</math> is a lower bound for the length of a Golomb ruler.  A long Golomb ruler will have gaps, that is, it will have distances which it cannot measure.  For example, the optimal Golomb ruler {0, 1, 4, 10, 12, 17} has length 17, but cannot measure lengths of 14 or 15.

== Wichmann rulers ==
Many optimal rulers are of the form W(r,s) = 1^r, r+1, (2r+1)^r, (4r+3)^s, (2r+2)^(r+1), 1^r, where a^b represents b segments of length a. Thus, if r = 1 and s = 2, then W(1,2) has (in order): <br/>
1 segment of length 1, <br/>
1 segment of length 2, <br/>
1 segment of length 3, <br/>
2 segments of length 7, <br/>
2 segments of length 4, <br/>
1 segment of length 1 <br/>

That gives the ruler {0, 1, 3, 6, 13, 20, 24, 28, 29}. The length of a Wichmann ruler is 4r(r+s+2)+3(s+1) and the number of marks is 4r+s+3. Note that not all Wichmann rulers are optimal and not all optimal rulers can be generated this way. None of the optimal rulers of length 1, 13, 17, 23 and 58 follow this pattern, but no optimal rulers with length greater than 68 are known that are not Wichmann rulers.

== Examples ==

The following are examples of minimal sparse rulers. Optimal rulers are highlighted. When there are too many to list, not all are included. Mirror images are not shown.

{| class="wikitable" border="1"
|-
! | Length
! | Marks
! | Number
! | Examples
! | List Form
! | Wichmann
|- style="background:#dedeff"
|| 1 || 2 || 1 || II || {0, 1} ||
|-
|| 2 || 3 || 1 || III || {0, 1, 2}
|- style="background:#dedeff"
|| 3 || 3 || 1 || II.I || {0, 1, 3} || W(0,0)
|-
|| 4 || 4 || 2 || III.I <br/> II.II || {0, 1, 2, 4} <br/> {0, 1, 3, 4}
|-
|| 5 || 4 || 2 || III..I <br/> II.I.I || {0, 1, 2, 5} <br/> {0, 1, 3, 5}
|- style="background:#dedeff"
|| 6 || 4 || 1 || II..I.I || {0, 1, 4, 6} || W(0,1)
|-
|| 7 || 5 || 6 || IIII...I <br/> III.I..I <br/> III..I.I <br/> II.I.I.I <br/> II.I..II <br/> II..II.I || {0, 1, 2, 3, 7} <br/> {0, 1, 2, 4, 7} <br/> {0, 1, 2, 5, 7} <br/> {0, 1, 3, 5, 7} <br/> {0, 1, 3, 6, 7} <br/> {0, 1, 4, 5, 7}
|-
|| 8 || 5 || 4 || III..I..I <br/> II.I...II <br/> II..I.I.I <br/> II...II.I || {0, 1, 2, 5, 8} <br/> {0, 1, 3, 7, 8} <br/> {0, 1, 4, 6, 8} <br/> {0, 1, 5, 6, 8}
|- style="background:#dedeff"
|| 9 || 5 || 2 || III...I..I <br/> II..I..I.I || {0, 1, 2, 6, 9} <br/> {0, 1, 4, 7, 9} || - <br/> W(0,2)
|-
|| 10 || 6 || 19 || IIII..I...I || {0, 1, 2, 3, 6, 10}
|-
|| 11 || 6 || 15 || IIII...I...I || {0, 1, 2, 3, 7, 11}
|-
|| 12 || 6 || 7 || IIII....I...I <br/> III...I..I..I <br/> II.I.I.....II <br/> II.I...I...II <br/> II..II....I.I <br/> II..I..I..I.I <br/> II.....II.I.I || {0, 1, 2, 3, 8, 12} <br/> {0, 1, 2, 6, 9, 12} <br/> {0, 1, 3, 5, 11, 12} <br/> {0, 1, 3, 7, 11, 12} <br/> {0, 1, 4, 5, 10, 12} <br/> {0, 1, 4, 7, 10, 12} <br/> {0, 1, 7, 8, 10, 12} || - <br/> - <br/> - <br/> - <br/> - <br/> W(0,3) <br/> -
|- style="background:#dedeff"
|| 13 || 6 || 3 || III...I...I..I <br/> II..II.....I.I <br/> II....I..I.I.I || {0, 1, 2, 6, 10, 13} <br/> {0, 1, 4, 5, 11, 13} <br/> {0, 1, 6, 9, 11, 13} ||
|-
|| 14 || 7 || 65 || IIIII....I....I || {0, 1, 2, 3, 4, 9, 14}
|-
|| 15 || 7 || 40 || II.I..I...I...II <br/> II..I..I..I..I.I || {0, 1, 3, 6, 10, 14, 15} <br/> {0, 1, 4, 7, 10, 13, 15} || W(1,0) <br/> W(0,4)
|-
|| 16 || 7 || 16 || IIII....I...I...I || {0, 1, 2, 3, 8, 12, 16}
|- style="background:#dedeff"
|| 17 || 7 || 6 || IIII....I....I...I <br/> III...I...I...I..I <br/> III.....I...I.I..I <br/> III.....I...I..I.I <br/> II..I.....I.I..I.I <br/> II......I..I.I.I.I || {0, 1, 2, 3, 8, 13, 17} <br/> {0, 1, 2, 6, 10, 14, 17} <br/> {0, 1, 2, 8, 12, 14, 17} <br/> {0, 1, 2, 8, 12, 15, 17} <br/> {0, 1, 4, 10, 12, 15, 17} <br/> {0, 1, 8, 11, 13, 15, 17} ||
|-
|| 18 || 8 || 250 || II..I..I..I..I..I.I || {0, 1, 4, 7, 10, 13, 16, 18} || W(0,5)
|-
|| 19 || 8 || 163 || IIIII....I....I....I || {0, 1, 2, 3, 4, 9, 14, 19}
|-
|| 20 || 8 || 75 || IIIII.....I....I....I || {0, 1, 2, 3, 4, 10, 15, 20}
|-
|| 21 || 8 || 33 || IIIII.....I.....I....I || {0, 1, 2, 3, 4, 10, 16, 21}
|-
|| 22 || 8 || 9 || IIII....I....I....I...I <br/> III.......I....I..I..II <br/> II.I.I........II.....II <br/> II.I..I......I...I...II <br/> II.I.....I.....I...II.I <br/> II..II......I.I.....I.I <br/> II....II..I.......I.I.I <br/> II....I..I......I.I.I.I <br/> II.....II........II.I.I || {0, 1, 2, 3, 8, 13, 18, 22} <br/> {0, 1, 2, 10, 15, 18, 21, 22} <br/> {0, 1, 3, 5, 14, 15, 21, 22} <br/> {0, 1, 3, 6, 13, 17, 21, 22} <br/> {0, 1, 3, 9, 15, 19, 20, 22} <br/> {0, 1, 4, 5, 12, 14, 20, 22} <br/> {0, 1, 6, 7, 10, 18, 20, 22} <br/> {0, 1, 6, 9, 16, 18, 20, 22} <br/> {0, 1, 7, 8, 17, 18, 20, 22} || - <br/> - <br/> - <br/> W(1,1) <br/> - <br/> - <br/> - <br/> - <br/> -
|- style="background:#dedeff"
|| 23 || 8 || 2 || III........I...I..I..I.I <br/> II..I.....I.....I.I..I.I || {0, 1, 2, 11, 15, 18, 21, 23} <br/> {0, 1, 4, 10, 16, 18, 21, 23} ||
|-
|| 24 || 9 || 472 || IIIIII......I.....I.....I || {0, 1, 2, 3, 4, 5, 12, 18, 24}
|-
|| 25 || 9 || 230 || IIIIII......I......I.....I || {0, 1, 2, 3, 4, 5, 12, 19, 25}
|-
|| 26 || 9 || 83 || IIIII.....I....I.....I....I || {0, 1, 2, 3, 4, 10, 15, 21, 26}
|-
|| 27 || 9 || 28 || IIIII.....I.....I.....I....I || {0, 1, 2, 3, 4, 10, 16, 22, 27}
|-
|| 28 || 9 || 6 || III..........I....I..I..I..II <br/> II.I.I.I..........II.......II <br/>  II.I..I..I......I......I...II <br/> II.I.....I.....I.....I...II.I <br/> II.....I...I........I..I.II.I <br/> II.......II..........II.I.I.I || {0, 1, 2, 13, 18, 21, 24, 27, 28} <br/> {0, 1, 3, 5, 7, 18, 19, 27, 28} <br/> {0, 1, 3, 6, 9, 16, 23, 27, 28} <br/> {0, 1, 3, 9, 15, 21, 25, 26, 28} <br/> {0, 1, 7, 11, 20, 23, 25, 26, 28} <br/> {0, 1, 9, 10, 21, 22, 24, 26, 28}
|- style="background:#dedeff"
|| 29 || 9 || 3 || III...........I...I..I..I..I.I <br/> II.I..I......I......I...I...II <br/> II..I.....I.....I.....I.I..I.I || {0, 1, 2, 14, 18, 21, 24, 27, 29} <br/> {0, 1, 3, 6, 13, 20, 24, 28, 29} <br/> {0, 1, 4, 10, 16, 22, 24, 27, 29} || - <br/> W(1,2) <br/> -
|-
|| 35 || 10 || 5 || III..............I...I..I..I..I..I.I <br/> II.I..I..I......I......I......I...II <br/> II.I..I..I.........I...I......I...II <br/> II..II..........I.I......I.I.....I.I <br/> II..I.....I.....I.....I.....I.I..I.I || {0, 1, 2, 17, 21, 24, 27, 30, 33, 35} <br/> {0, 1, 3, 6, 9, 16, 23, 30, 34, 35} <br/> {0, 1, 3, 6, 9, 19, 23, 30, 34, 35} <br/> {0, 1, 4, 5, 16, 18, 25, 27, 33, 35} <br/> {0, 1, 4, 10, 16, 22, 28, 30, 33, 35}
|- style="background:#dedeff"
|| 36 || 10 || 1 || II.I..I......I......I......I...I...II || {0, 1, 3, 6, 13, 20, 27, 31, 35, 36} || W(1,3)
|- style="background:#dedeff"
|| 43 || 11 || 1 || II.I..I......I......I......I......I...I...II || {0, 1, 3, 6, 13, 20, 27, 34, 38, 42, 43} || W(1,4)
|-
|| 46 || 12 || 342 || III..I....I....I..........I.....I.....I.....III || {0, 1, 2, 5, 10, 15, 26, 32, 38, 44, 45, 46} || W(2,1)
|- style="background:#dedeff"
|| 50 || 12 || 2 || IIII...................I....I...I...I...I...I..I..I <br/> II.I..I.....I......I......I......I......I...I...II || {0, 1, 2, 3, 23, 28, 32, 36, 40, 44, 47, 50} <br> {0, 1, 3, 6, 13, 20, 27, 34, 41, 45, 49, 50} || - <br/> W(1,5)
|-
|| 57 || 13 || 12 || III..I....I....I..........I..........I.....I.....I.....III <br/> II.I..I......I......I......I......I......I......I...I...II || {0, 1, 2, 5, 10, 15, 26, 37, 43, 49, 55, 56, 57} <br/> {0, 1, 3, 6, 13, 20, 27, 34, 41, 48, 52, 56, 57} || W(2,2) <br> W(1,6)
|- style="background:#dedeff"
|| 58 || 13 || 6 || IIII.......................I....I...I...I...I...I...I..I..I <br/> III...I.I........I........I........I........I..I......I..II <br/> III.....I......II.........I.........I.........I..I...I.I..I <br/> II.I..I..........I..I......I.......I.........I...I...I...II <br/> II.I..I..........I......I..I..........I......I...I...I...II <br/> II...I..I...I........I........I........I........I....II.I.I || {0, 1, 2, 3, 27, 32, 36, 40, 44, 48, 52, 55, 58} <br/> {0, 1, 2, 6, 8, 17, 26, 35, 44, 47, 54, 57, 58} <br/> {0, 1, 2, 8, 15, 16, 26, 36, 46, 49, 53, 55, 58} <br/> {0, 1, 3, 6, 17, 20, 27, 35, 45, 49, 53, 57, 58} <br/> {0, 1, 3, 6, 17, 24, 27, 38, 45, 49, 53, 57, 58} <br/> {0, 1, 5, 8, 12, 21, 30, 39, 48, 53, 54, 56, 58} ||
|- style="background:#dedeff"
|| 68 || 14 || 2 || III..I....I....I..........I..........I..........I.....I.....I.....III <br/> III.....I......II.........I.........I.........I.........I..I...I.I..I || {0, 1, 2, 5, 10, 15, 26, 37, 48, 54, 60, 66, 67, 68} <br/> {0, 1, 2, 8, 15, 16, 26, 36, 46, 56, 59, 63, 65, 68} || W(2,3) <br/> -
|- style="background:#dedeff"
|| 79 || 15 || 1 || III..I....I....I..........I..........I..........I..........I.....I.....I.....III || {0, 1, 2, 5, 10, 15, 26, 37, 48, 59, 65, 71, 77, 78, 79} || W(2,4)
|- style="background:#dedeff"
|| 90 || 16 || 1 || <small>III..I....I....I..........I..........I..........I..........I..........I.....I.....I.....III</small> || {0, 1, 2, 5, 10, 15, 26, 37, 48, 59, 70, 76, 82, 88, 89, 90} || W(2,5)
|- style="background:#dedeff"
|| 101 || 17 || 1 || <small>III..I....I....I..........I..........I..........I..........I..........I..........I.....I.....I.....III</small> || {0,1,2,5,10,15,26,37,48,59,70,81,87,93,99,100,101} || W(2,6)
|- style="background:#dedeff"
|| 112 || 18 || 1 || || {0,1,2,5,10,15,26,37,48,59,70,81,92,98,104,110,111,112} || W(2,7)
|- style="background:#dedeff"
|| 123 || 19 || 2 || || <small>{0,1,2,3,7,14,21,28,43,58,73,88,96,104,112,120,121,122,123} <br> {0,1,2,5,10,15,26,37,48,59,70,81,92,103,109,115,121,122,123}</small> || W(3,4) <br> W(2,8)
|- style="background:#dedeff"
|| 138 || 20 || 1 || || <small>{0,1,2,3,7,14,21,28,43,58,73,88,103,111,119,127,135,136,137,138}</small> || W(3,5)
|}

==Incomplete sparse rulers==
A few incomplete rulers can fully measure up to a longer distance than an optimal sparse ruler with the same number of marks. <math>\{0, 2, 7, 14, 15, 18, 24\}</math>, <math>\{0, 2, 7, 13, 16, 17, 25\}</math>, <math>\{0, 5, 7, 13, 16, 17, 31\}</math>, and <math>\{0, 6, 10, 15, 17, 18, 31\}</math> can each measure up to 18, while an optimal sparse ruler with 7 marks can measure only up to 17. The table below lists these rulers, up to rulers with 13 marks. Mirror images are not shown. Rulers that can fully measure up to a longer distance than any shorter ruler with the same number of marks are highlighted.

{| class="wikitable"
|-
! Marks !! Length !! Measures up to !! Ruler
|- style="background:#dedeff"
| 7 || 24 || 18 || {0, 2, 7, 14, 15, 18, 24}
|-
| 7 || 25 || 18 || {0, 2, 7, 13, 16, 17, 25}
|-
| 7 || 31 || 18 || {0, 5, 7, 13, 16, 17, 31}
|-
| 7 || 31 || 18 || {0, 6, 10, 15, 17, 18, 31}
|- style="background:#dedeff"
| 8 || 39 || 24 || {0, 8, 15, 17, 20, 21, 31, 39}
|- style="background:#dedeff"
| 10 || 64 || 37 || {0, 7, 22, 27, 28, 31, 39, 41, 57, 64}
|-
| 10 || 73 || 37 || {0, 16, 17, 28, 36, 42, 46, 49, 51, 73}
|- style="background:#dedeff"
| 11 || 68 || 44 || {0, 7, 10, 27, 29, 38, 42, 43, 44, 50, 68}
|- style="background:#dedeff"
| 11 || 91 || 45 || {0, 18, 19, 22, 31, 42, 48, 56, 58, 63, 91}
|- style="background:#dedeff"
| 12 || 53 || 51 || {0, 2, 3, 6, 9, 17, 25, 33, 41, 46, 51, 53}
|-
| 12  || 60 || 51 || {0, 5, 9, 13, 19, 26, 33, 48, 49, 50, 51, 60}
|-
| 12 || 73 || 51 || {0, 2, 3, 10, 17, 23, 35, 42, 46, 47, 51, 73}
|-
| 12 || 75 || 51 || {0, 2, 10, 13, 29, 33, 36, 45, 50, 51, 57, 75}
|-
| 12 || 82 || 51 || {0, 8, 28, 31, 34, 38, 45, 47, 49, 50, 74, 82}
|-
| 12 || 83 || 51 || {0, 2, 10, 24, 25, 29, 36, 42, 45, 73, 75, 83}
|-
| 12 || 85 || 51 || {0, 8, 10, 19, 35, 41, 42, 47, 55, 56, 59, 85}
|-
| 12 || 87 || 51 || {0, 12, 24, 26, 37, 39, 42, 43, 46, 47, 75, 87}
|- style="background:#dedeff"
| 13 || 61 || 59 || {0, 2, 3, 6, 9, 17, 25, 33, 41, 49, 54, 59, 61}
|-
| 13 || 69 || 59 || {0, 6, 10, 15, 22, 30, 38, 55, 56, 57, 58, 59, 69}
|-
| 13 || 69 || 59 || {0, 6, 11, 15, 22, 30, 38, 55, 56, 57, 58, 59, 69}
|-
| 13 || 82 || 59 || {0, 4, 5, 9, 25, 27, 39, 42, 50, 53, 56, 63, 82}
|-
| 13 || 83 || 59 || {0, 1, 2, 24, 34, 36, 38, 43, 51, 54, 57, 82, 83}
|-
| 13 || 88 || 59 || {0, 1, 3, 9, 16, 26, 36, 40, 47, 54, 58, 59, 88}
|-
| 13 || 88 || 59 || {0, 1, 5, 29, 34, 36, 47, 48, 50, 56, 58, 73, 88}
|-
| 13 || 90 || 59 || {0, 7, 12, 16, 37, 38, 43, 55, 56, 57, 58, 66, 90}
|-
| 13 || 91 || 59 || {0, 5, 9, 12, 16, 32, 38, 42, 55, 56, 57, 63, 91}
|-
| 13 || 92 || 59 || {0, 6, 10, 13, 25, 34, 39, 54, 55, 56, 57, 65, 92}
|-
| 13 || 94 || 59 || {0, 1, 3, 16, 28, 37, 45, 48, 54, 55, 59, 78, 94}
|-
| 13 || 95 || 59 || {0, 4, 32, 37, 38, 40, 48, 53, 54, 56, 63, 83, 95}
|-
| 13 || 96 || 59 || {0, 3, 7, 27, 37, 39, 50, 55, 56, 58, 72, 81, 96}
|-
| 13 || 101 || 59 || {0, 4, 24, 37, 43, 45, 52, 54, 55, 59, 77, 81, 101}
|-
| 13 || 108 || 59 || {0, 8, 17, 40, 50, 53, 64, 65, 69, 71, 91, 99, 108}
|- style="background:#dedeff"
| 13 || 113 || 61 || {0, 6, 22, 36, 45, 47, 57, 60, 64, 65, 91, 97, 113}
|-
| 13 || 133 || 60 || {0, 26, 29, 40, 42, 46, 67, 74, 79, 89, 97, 98, 133}
|}

==See also==
* [[gauge block]]
* [[Golomb ruler]]
* [[Perfect ruler]]

== References ==
* http://www.luschny.de/math/rulers/prulers.html
* http://oeis.org/wiki/User:Peter_Luschny/PerfectRulers
* http://www.iwriteiam.nl/Ha_sparse_rulers.html
* http://www.maa.org/editorial/mathgames/mathgames_11_15_04.html
* http://www.contestcen.com/scale.htm
* http://members.cox.net/wnmyers/sparse_rulers.txt

[[Category:Number theory]]
[[Category:Combinatorics]]