login
A299481
Irregular triangle read by rows in which row n lists the odd divisors of n in decreasing order together with the even divisors of n in increasing order.
7
1, 1, 2, 3, 1, 1, 2, 4, 5, 1, 3, 1, 2, 6, 7, 1, 1, 2, 4, 8, 9, 3, 1, 5, 1, 2, 10, 11, 1, 3, 1, 2, 4, 6, 12, 13, 1, 7, 1, 2, 14, 15, 5, 3, 1, 1, 2, 4, 8, 16, 17, 1, 9, 3, 1, 2, 6, 18, 19, 1, 5, 1, 2, 4, 10, 20, 21, 7, 3, 1, 11, 1, 2, 22, 23, 1, 3, 1, 2, 4, 6, 8, 12, 24, 25, 5, 1, 13, 1, 2, 26, 27, 9, 3, 1
OFFSET
1,3
COMMENTS
Consider the diagram with overlapping periodic curves that appears in the Links section (figure 1). The number of curves that contain the point [n,0] equals the number of divisors of n. The simpler interpretation of the diagram is that the curve of diameter d represents the divisor d of n. Now here we introduce a new interpretation: the curve of diameter d that contains the point [n,0] represents the divisor c of n, where c = n/d. This version of the model has the property that each odd quadrant centered at [n,0] contains the curves that represent the even divisors of n, and each even quadrant centered at [n,0] contains the curves that represent the odd divisors of n.
We can find the n-th row of the triangle as follows:
Consider only the semicircumferences that contain the point [n,0].
In the second quadrant from bottom to top we can see the curves that represent the odd divisors of n in decreasing order. Also we can see these curves in the fourth quadrant from top to bottom.
Then, if n is an even number, in the first quadrant from top to bottom we can see the curves that represent the even divisors of n in increasing order. Also we can see these curves in the third quadrant from bottom to top (see example).
Sequences of the same family are shown below:
-----------------------------------
Triangle Order of divisors of n
-----------------------------------
This seq. odd v t.w. even ^
A299483 odd ^ t.w. even v
A319844 even v t.w. odd ^
A319845 even ^ t.w. odd v
A319846 odd v t.w. even v
A319847 odd ^ t.w. even ^
A319848 even v t.w. odd v
A319849 even ^ t.w. odd ^
-----------------------------------
In the above table we have that:
"even v" means "even divisors of n in decreasing order".
"even ^" means "even divisors of n in increasing order".
"odd v" means "odd divisors of n in decreasing order".
"odd ^" means "odd divisors of n in increasing order".
"t.w." means "together with".
EXAMPLE
Triangle begins:
1;
1, 2;
3, 1;
1, 2, 4;
5, 1;
3, 1, 2, 6;
7, 1;
1, 2, 4, 8;
9, 3, 1;
5, 1, 2, 10;
11, 1;
3, 1, 2, 4, 6, 12;
13, 1;
7, 1, 2, 14;
15, 5, 3, 1;
1, 2, 4, 8, 16;
17, 1;
9, 3, 1, 2, 6, 18;
19, 1;
5, 1, 2, 4, 10, 20;
21, 7, 3, 1;
11, 1, 2, 22;
23, 1;
3, 1, 2, 4, 6, 8, 12, 24;
25, 5, 1;
13, 1, 2, 26;
27, 9, 3, 1;
7, 1, 2, 4, 14, 28;
...
For n = 12 the divisors of 12 are [1, 2, 3, 4, 6, 12]. The odd divisors of 12 in decreasing order are [3, 1], and the even divisors of 12 in increasing order are [2, 4, 6, 12], so the 12th row of triangle is [3, 1, 2, 4, 6, 12].
On the other hand, consider the diagram that appears in the Links section (figure 1). Then consider only the semicircumferences that contain the point [12,0]. In the second quadrant, from bottom to top, we can see the curves with diameters [4, 12]. Also we can see these curves in the fourth quadrant from top to bottom. The associated numbers c = 12/d are [3, 1] respectively. These are the odd divisors of 12 in decreasing order. Then, in the first quadrant, from top to bottom, we can see the curves with diameters [6, 3, 2, 1]. Also we can see these curves in the third quadrant from bottom to top. The associated numbers c = 12/d are [2, 4, 6, 12] respectively. These are the even divisors of n in increasing order. Finally all numbers c obtained are [3, 1, 2, 4, 6, 12] equaling the 12th row of triangle.
PROG
(PARI) row(n) = my(d=divisors(n)); concat(Vecrev(select(x->(x%2), d)), select(x->!(x%2), d));
lista(nn) = {for (n=1, nn, my(r = row(n)); for (k=1, #r, print1(r[k], ", ")); ); } \\ Michel Marcus, Jan 17 2019
CROSSREFS
Row sums give A000203.
Row n has length A000005(n).
Alternating borders give A000027.
Right border gives A124625 without its first two terms.
Sequence in context: A123338 A152735 A249128 * A304738 A046226 A054722
KEYWORD
nonn,tabf
AUTHOR
Omar E. Pol, Feb 10 2018
STATUS
approved