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A288813 Irregular triangle read by rows: T(m, k) is the list of squarefree numbers A002110(m) < t < 2*A002110(m) such that A001221(t) = m. 4
3, 10, 42, 330, 390, 2730, 3570, 3990, 4290, 39270, 43890, 46410, 51870, 53130, 570570, 690690, 746130, 870870, 881790, 903210, 930930, 1009470, 11741730, 13123110, 14804790, 15825810, 16546530, 17160990, 17687670, 18888870, 281291010, 300690390, 340510170 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(n) = terms t of row m of A288784 such that A002110(m) < t < 2*A002110(m).

The only odd term is 3; the only other term not ending in 10, 30, 70, or 90 in decimal is 42.

All terms t in row m have A001221(t) = m and at least one prime q coprime to t such that q < A006530(t).

Consider "tier" m and primorial p_m# = A002110(m), let "distension" i = pi(A006530(T(m, k))) - m and let "depth" j = m - pi(A053669(T(m, k))) + 1. Distension is the difference in the index of gpf(T(m, k)) and pi(m), while depth is the difference between the index of the least prime totative of T(m, k) and pi(m) + 1. We can calculate the maximum distension i given m and j via i_max = A020900(m - j + 1) - m - j + 1. This enables us to use permutations of 0 and 1 values in the notation A054841 and produce a(n) with some efficiency.

The most efficient method of generating a(n) is via f(x) = A287352(x), i.e., subtracting 1 from all values in row x of A287352. We use a pointer variable to direct increment on f(p_m#) = a constant array of m 1's, until we have exhausted producing terms p_m# < t < 2*p_m#. This enables the generation of T(m, k) for 1 <= m <= 100.

LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..14936 (Rows 1 <= m <= 36)

Eric Weisstein's World of Mathematics, Primorial

Eric Weisstein's World of Mathematics, Squarefree

Michael De Vlieger, Relations between A288813, A288784, A002110, and A244052, including prime decompositions of terms of a(n) and all code used to generate the tables.

EXAMPLE

Triangle begins:

n     a(n)

1:       3

2:      10

3:      42

4:     330     390

5:    2730    3570    3990    4290

6:   39270   43890   46410   51870   53130

7:  570570  690690  746130  870870  881790  903210  930930  1009470

       ...

MATHEMATICA

Table[Function[P, Select[Range[P + 1, 2 P - 1], And[SquareFreeQ@ #, PrimeOmega@ # == n] &]]@ Product[Prime@ i, {i, n}], {n, 7}] // Flatten (* Michael De Vlieger, Jun 24 2017 *)

f[n_] := Block[{P = Product[Prime@ i, {i, n}], lim, k = 1, c, w = ConstantArray[1, n]}, lim = 2 P; Sort@ Reap[Do[w = If[k == 1, MapAt[# + 1 &, w, -k], Join[Drop[MapAt[# + 1 &, w, -k], -k + 1], ConstantArray[1, k - 1]]]; c = Times @@ Map[If[# == 0, 1, Prime@ #] &, Accumulate@ w]; If[c < lim, Sow[c]; k = 1, If[k == n, Break[], k++]], {i, Infinity}] ][[-1, 1]] ]; Array[f, 9] // Flatten (* Michael De Vlieger, Jun 28 2017, faster *)

PROG

(PARI) primo(n) = prod(i=1, n, prime(i));

row(n) = my(vrow = []); for (j=primo(n)+1, 2*primo(n)-1, if (issquarefree(j) && (omega(j)==n), vrow = concat(vrow, j))); vrow;

tabf(nn) = for (n=1, nn, print(row(n))); \\ Michel Marcus, Jun 29 2017

CROSSREFS

Cf. A001221, A002110, A020900, A288784.

Sequence in context: A185621 A190657 A143523 * A306237 A042545 A203266

Adjacent sequences:  A288810 A288811 A288812 * A288814 A288815 A288816

KEYWORD

nonn,tabf,easy

AUTHOR

Michael De Vlieger, Jun 24 2017

STATUS

approved

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Last modified April 19 08:34 EDT 2019. Contains 322241 sequences. (Running on oeis4.)