A381440 Irregular triangle read by rows where row k is the Look-and-Say partition of the prime indices of n.

1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 4, 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1

Offset

1,4

Comments

Row lengths are A066328.

A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798.

The Look-and-Say partition of a multiset or partition y is obtained by interchanging parts with multiplicities. For example, starting with (3,2,2,1,1) we get (2,2,2,1,1,1), the multiset union of ((1,1,1),(2,2),(2)).

The conjugate of a Look-and-Say partition is a section-sum partition; see A381431, union A381432, count A239455.

Links

Table of n, a(n) for n=1..80.

Example

The prime indices of 24 are (2,1,1,1), with Look-and-Say partition (3,1,1), so row 24 is (3,1,1).
The prime indices of 36 are (2,2,1,1), with Look-and-Say partition (2,2,2), so row 36 is (2,2,2).
Triangle begins:
   1: (empty)
   2: 1
   3: 1 1
   4: 2
   5: 1 1 1
   6: 1 1 1
   7: 1 1 1 1
   8: 3
   9: 2 2
  10: 1 1 1 1
  11: 1 1 1 1 1
  12: 2 1 1
  13: 1 1 1 1 1 1
  14: 1 1 1 1 1
  15: 1 1 1 1 1
  16: 4
  17: 1 1 1 1 1 1 1
  18: 2 2 1
  19: 1 1 1 1 1 1 1 1

Mathematica

Table[Sort[Join@@Cases[FactorInteger[n], {p_, k_}:>ConstantArray[k, PrimePi[p]]]]//Reverse, {n, 30}]

Crossrefs

Heinz numbers are A048767 (union A351294, complement A351295, fixed A048768, A217605).

First part in each row is A051903, conjugate A066328.

Last part in each row is A051904, conjugate A381437 (counted by A381438).

Row sums are A056239.

Row lengths are A066328.

Partitions of this type are counted by A239455, complement A351293.

The conjugate is A381436, Heinz numbers A381431 (union A381432, complement A381433).

Rows appearing only once have Heinz numbers A381540, more than once A381541.

A000040 lists the primes.

A003963 gives product of prime indices.

A055396 gives least prime index, greatest A061395.

A056239 adds up prime indices, row sums of A112798, counted by A001222.

A122111 represents conjugation in terms of Heinz numbers.

Set multipartitions: A050320, A089259, A116540, A270995, A296119, A318360, A318361.

Partition ideals: A300383, A317141, A381078, A381441, A381452, A381454.

Cf. A000720, A003557, A047966, A071178, A091602, A116861, A130091, A181819, A212166, A238744, A380955.

Sequence in context: A161606 A300362 A248145 * A171398 A113607 A351352

Adjacent sequences: A381437 A381438 A381439 * A381441 A381442 A381443

Keyword

nonn,tabf,new

Author

Gus Wiseman, Feb 28 2025

Status

approved