GCD

Chan shows that these top nodes and children visit all rationals X/Y with X,Y one odd, one even. But the X,Y are not in least terms, they may have a power-of-3 common factor GCD(X,Y)=3^m for some m.

The GCD is unchanged in the first and third children. The middle child GCD might gain an extra factor 3. This means the power is at most the number of middle legs taken, which is the count of ternary 1-digits of its position across the row.

    GCD(X,Y) = 3^m
    m <= count ternary 1-digits of N+1, excluding high digit

As per "N Start" below, N+1 in ternary has high digit 1 or 2 which indicates the tree root. Ignoring that high digit gives an offset into the row of that tree and the digits are 0,1,2 for left,middle,right.

For example the first GCD is at N=9 with X=6,Y=9 common factor GCD=3. N+1=10="101" ternary, which without the high digit is "01" which has a single "1" so GCD <= 3^1. The mirror image of this point is X=9,Y=6 at N=24 and there N+1=24+1=25="221" ternary which without the high digit is "21" with a single 1-digit likewise.

For various points the power m is equal to the count of 1-digits.

k Parameter

The "k => $integer" parameter controls the number of children and top nodes. There are k-1 top nodes and each node has k children. The top nodes are

    k odd, k-1 many tops, with h=ceil(k/2)
    1/2  2/3  3/4  ... (h-1)/h       h/(h-1) ...  4/3  3/2  2/1
    k even, k-1 many tops, with h=k/2
    1/2  2/3  3/4  ... (h-1)/h  h/h  h/(h-1) ...  4/3  3/2  2/1

Notice the list for k odd or k even is the same except that for k even there's an extra middle term h/h. The first few tops are as follows. The list in each row is spread to show how successive bigger h adds terms in the middle.

     k                 X/Y top nodes
    ---    ---------------------------------
    k=2                   1/1
    k=3              1/2       2/1
    k=4              1/2  2/2  2/1
    k=5         1/2  2/3       3/2  2/1
    k=6         1/2  2/3  3/3  3/2  2/1
    k=7    1/2  2/3  3/4       4/3  3/2  2/1
    k=8    1/2  2/3  3/4  4/4  4/3  3/2  2/1

As X,Y coordinates these tops are a run up along X=Y-1 and back down along X=Y+1, with a middle X=Y point if k even. For example,

      7 |                         5         k=13 top nodes N=0 to N=11
      6 |                     4       6        total 12 top nodes
      5 |                 3       7
      4 |             2       8
      3 |         1       9
      2 |     0      10
      1 |        11
    Y=0 |
        +------------------------------
        X=0   1   2   3   4   5   6   7
                                            k=14 top nodes N=0 to N=12
      7 |                         5   6        total 13 top nodes
      6 |                     4       7
      5 |                 3       8         N=6 is the 7/7 middle term
      4 |             2       9
      3 |         1      10
      2 |     0      11
      1 |        12
    Y=0 |
        +------------------------------
        X=0   1   2   3   4   5   6   7

Each node has k children. The formulas for the children can be seen from sample cases k=5 and k=6. A node X/Y descends to

    k=5                     k=6
    1X+0Y / 2X+1Y           1X+0Y / 2X+1Y
    2X+1Y / 3X+2Y           2X+1Y / 3X+2Y
    3X+2Y / 2X+3Y           3X+2Y / 3X+3Y
    2X+3Y / 1X+2Y           3X+3Y / 2X+3Y
    1X+2Y / 0X+1Y           2X+3Y / 1X+2Y
                            1X+2Y / 0X+1Y

The coefficients of X and Y run up to h=ceil(k/2) starting from either 0, 1 or 2 and ending 2, 1 or 0. When k is even there's two h coeffs in the middle. When k is odd there's just one. The resulting tree for example with k=4 is

    k=4
          1/2              2/2               2/1
       /       \        /        \        /       \
    1/4 4/6 6/5 5/2  2/6 6/8 8/6 6/2   2/5 5/6 6/4 4/1

Chan shows that this combination of top nodes and children visits

    if k odd:    rationals X/Y with X,Y one odd, one even
                  possible GCD(X,Y)=k^m for some integer m
    if k even:   all rationals X/Y
                  possible GCD(X,Y) a divisor of (k/2)^m

When k odd GCD(X,Y) is a power of k, so for example as described above k=3 gives GCD=3^m. When k even GCD(X,Y) is a divisor of (k/2)^m but not necessarily a full such power. For example with k=12 the first such non-power GCD is at N=17 where X=16,Y=18 has GCD(16,18)=2 which is only a divisor of k/2=6, not a power of 6.

N Start

The "n_start => $n" option can select a different initial N. The tree structure is unchanged, just the numbering shifted. As noted above the default Nstart=0 corresponds to powers in a generating function.

"n_start=>1" makes the numbering correspond to digits of N written in base-k. For example k=10 corresponds to N written in decimal,

    N=1 to 9                1/2    ...  ...    2/1
    N=10 to 99          1/4 4/7  ...      ...  7/4 4/1
    N=100 to 999    1/6 6/11   ...          ...   11/6 6/1

In general "n_start=>1" makes the tree

    N written in base-k digits
     depth = numdigits(N)-1
     NdepthStart = k^depth
                 = 100..000 base-k, high 1 in high digit position of N
     N-NdepthStart = position across whole row of all top trees

And the high digit of N selects which top-level tree the given N is under, so

    N written in base-k digits
     top tree = high digit of N
                (1 to k, selecting the k-1 many top nodes)
     Nrem = digits of N after the highest
          = position across row within the high-digit tree
     depth = numdigits(Nrem)       # top node depth=0
           = numdigits(N)-1

Diatomic Sequence

Chan shows that each denominator Y becomes the numerator X in the next point. The last Y of a row becomes the first X of the next row. This is a generalization of Stern's diatomic sequence and of the Calkin-Wilf tree of rationals. (See Math::NumSeq::SternDiatomic and "Calkin-Wilf Tree" in Math::PlanePath::RationalsTree.)

The case k=2 is precisely the Calkin-Wilf tree. There's just one top node 1/1, being the even k "middle" form h/h with h=k/2=1 as described above. Then there's two children of each node (the "middle" pair of the even k case),

    k=2, Calkin-Wilf tree
                     X/Y
                   /     \
    (1X+0Y)/(1X+1Y)       (1X+1Y)/(0X+1Y)
       = X/(X+Y)             = (X+Y)/Y

Tree Methods

Each point has k children, so the path is a complete k-ary tree.

"@n_children = $path->tree_n_children($n)"

Return the children of $n, or an empty list if "$n < n_start()", ie. before the start of the path.

"$num = $path->tree_n_num_children($n)"

Return k, since every node has k children. Or return "undef" if "$n < n_start()", ie. before the start of the path.

"$n_parent = $path->tree_n_parent($n)"

Return the parent node of $n, or "undef" if $n has no parent either because it's a top node or before "n_start()".

"$n_root = $path->tree_n_root ($n)"

Return the N which is root node of $n.

"$depth = $path->tree_n_to_depth($n)"

Return the depth of node $n, or "undef" if there's no point $n. The tree tops are depth=0, then their children depth=1, etc.

"$n = $path->tree_depth_to_n($depth)"

"$n = $path->tree_depth_to_n_end($depth)"

Return the first or last N at tree level $depth in the path. The top of the tree is depth=0.

Tree Descriptive Methods

"$num = $path->tree_num_roots ()"

Return the number of root nodes in $path, which is k-1. For example the default k=3 return 2 as there are two root nodes.

"@n_list = $path->tree_root_n_list ()"

Return a list of the N values which are the root nodes of $path. This is "n_start()" through "n_start()+k-2" inclusive, being the first k-1 many points. For example in the default k=2 and Nstart=0 the return is two values "(0,1)".

"$num = $path->tree_num_children_minimum()"

"$num = $path->tree_num_children_maximum()"

Return k since every node has k many children, making that both the minimum and maximum.

"$bool = $path->tree_any_leaf()"

Return false, since there are no leaf nodes in the tree.

N Children

For the default k=3 the children are

    3N+2, 3N+3, 3N+4        n_start=0

If "n_start=>1" then instead

    3N, 3N+1, 3N+2                  n_start=1

For this "n_start=1" the children are found by appending an extra ternary digit, or base-k digit for arbitrary k.

    k*N, k*N+1, ... , k*N+(k-1)     n_start=1

In general for k and Nstart the children are

    kN - (k-1)*(Nstart-1)  + 0
      ...
    kN - (k-1)*(Nstart-1)  + k-1

N Parent

The parent node reverses the children calculation above. The simplest case is "n_start=1" where it's a division to remove the lowest base-k digit

    parent = floor(N/k)       when n_start=1

For other "n_start" adjust before and after to an "n_start=1" basis,

    parent = floor((N-(Nstart-1)) / k) + Nstart-1

For example in the default k=0 Nstart=1 the parent of N=3 is floor((3-(1-1))/3)=1.

The post-adjustment can be worked into the formula with (k-1)*(Nstart-1) similar to the children above,

    parent = floor((N + (k-1)*(Nstart-1)) / k)

But the first style is more convenient to compare to see that N is past the top nodes and therefore has a parent.

    N-(Nstart-1) >= k      to check N is past top-nodes

N Root

As described under "N Start" above, if Nstart=1 then the tree root is simply the most significant base-k digit of N. For other Nstart an adjustment is made to N=1 style and back again.

    adjust = Nstart-1
    Nroot(N) = high_base_k_digit(N-adjust) + adjust

N to Depth

The structure of the tree means

    depth = floor(logk(N+1))    for n_start=0

For example if k=3 then all of N=8 through N=25 inclusive have depth=floor(log3(N+1))=2. With an "n_start" it becomes

    depth = floor(logk(N-(Nstart-1)))

"n_start=1" is the simplest case, being the length of N written in base-k digits.

    depth = floor(logk(N))     for n_start=1