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A319845 Irregular triangle read by rows in which row n lists the even divisors of n in increasing order together with the odd divisors of n in decreasing order. 7
1, 2, 1, 3, 1, 2, 4, 1, 5, 1, 2, 6, 3, 1, 7, 1, 2, 4, 8, 1, 9, 3, 1, 2, 10, 5, 1, 11, 1, 2, 4, 6, 12, 3, 1, 13, 1, 2, 14, 7, 1, 15, 5, 3, 1, 2, 4, 8, 16, 1, 17, 1, 2, 6, 18, 9, 3, 1, 19, 1, 2, 4, 10, 20, 5, 1, 21, 7, 3, 1, 2, 22, 11, 1, 23, 1, 2, 4, 6, 8, 12, 24, 3, 1, 25, 5, 1, 2, 26, 13, 1, 27, 9, 3, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

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 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].

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.

Then, 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 (see example).

Sequences of the same family are shown below:

-----------------------------------

Triangle    Order of divisors of n

-----------------------------------

A299481      odd  v   t.w.  even ^

A299483      odd  ^   t.w.  even v

A319844      even v   t.w.  odd  ^

This seq.    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".

LINKS

Table of n, a(n) for n=1..95.

Omar E. Pol, Figure 1: Geometric model of divisors with periodic curves (for n = 1..16), figure 2: Upper part, figure 3: Lower part upside down.

Index entries for sequences related to divisors of numbers

EXAMPLE

Triangle begins:

   1;

   2,  1;

   3,  1;

   2,  4,  1;

   5,  1;

   2,  6,  3,  1;

   7,  1;

   2,  4,  8,  1;

   9,  3,  1;

   2, 10,  5,  1;

  11,  1;

   2,  4,  6, 12,  3,  1;

  13,  1;

   2, 14,  7,  1;

  15,  5,  3,  1;

   2,  4,  8, 16,  1;

  17,  1;

   2,  6, 18,  9,  3,  1;

  19,  1;

   2,  4, 10, 20,  5,  1;

  21,  7,  3,  1;

   2, 22, 11,  1;

  23,  1;

   2,  4,  6,  8, 12, 24,  3,  1;

  25,  5,  1;

   2, 26, 13,  1;

  27,  9,  3,  1;

   2,  4, 14, 28,  7,  1;

...

For n = 12 the divisors of 12 are [1, 2, 3, 4, 6, 12]. The even divisors of 12 in increasing order are [2, 4, 6, 12], and the odd divisors of 12 in decreasing order are [3, 1], so the 12th row of triangle is [2, 4, 6, 12, 3, 1].

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 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. Then, 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. Finally all numbers c obtained are [2, 4, 6, 12, 3, 1] equaling the 12th row of triangle.

PROG

(PARI) row(n) = my(d=divisors(n)); concat(select(x->!(x%2), d), Vecrev(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).

Right border gives A000012.

Other permutations of A027750 are A056538, A210959, A299481, A299483, A319844, A319846, A319847, A319848, A319849.

Cf. A001227, A183063, A299480, A299485.

Sequence in context: A275055 A254679 A275280 * A319847 A087295 A175344

Adjacent sequences:  A319842 A319843 A319844 * A319846 A319847 A319848

KEYWORD

nonn,tabf

AUTHOR

Omar E. Pol, Sep 29 2018

STATUS

approved

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Last modified February 18 19:50 EST 2019. Contains 320262 sequences. (Running on oeis4.)