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A339561 Products of distinct squarefree semiprimes. 25
1, 6, 10, 14, 15, 21, 22, 26, 33, 34, 35, 38, 39, 46, 51, 55, 57, 58, 60, 62, 65, 69, 74, 77, 82, 84, 85, 86, 87, 90, 91, 93, 94, 95, 106, 111, 115, 118, 119, 122, 123, 126, 129, 132, 133, 134, 140, 141, 142, 143, 145, 146, 150, 155, 156, 158, 159, 161, 166 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

First differs from A320911 in lacking 36.

A squarefree semiprime (A006881) is a product of any two distinct prime numbers.

The following are equivalent characteristics for any positive integer n:

(1) the prime factors of n can be partitioned into distinct strict pairs (a set of edges);

(2) n can be factored into distinct squarefree semiprimes;

(3) the prime signature of n is graphical.

LINKS

Table of n, a(n) for n=1..59.

Eric Weisstein's World of Mathematics, Graphical partition.

FORMULA

A339561 \/ A320894 = A028260.

EXAMPLE

The sequence of terms together with their prime indices begins:

      1: {}        55: {3,5}         91: {4,6}

      6: {1,2}     57: {2,8}         93: {2,11}

     10: {1,3}     58: {1,10}        94: {1,15}

     14: {1,4}     60: {1,1,2,3}     95: {3,8}

     15: {2,3}     62: {1,11}       106: {1,16}

     21: {2,4}     65: {3,6}        111: {2,12}

     22: {1,5}     69: {2,9}        115: {3,9}

     26: {1,6}     74: {1,12}       118: {1,17}

     33: {2,5}     77: {4,5}        119: {4,7}

     34: {1,7}     82: {1,13}       122: {1,18}

     35: {3,4}     84: {1,1,2,4}    123: {2,13}

     38: {1,8}     85: {3,7}        126: {1,2,2,4}

     39: {2,6}     86: {1,14}       129: {2,14}

     46: {1,9}     87: {2,10}       132: {1,1,2,5}

     51: {2,7}     90: {1,2,2,3}    133: {4,8}

For example, the number 1260 can be factored into distinct squarefree semiprimes in two ways, (6*10*21) or (6*14*15), so 1260 is in the sequence. The number 69300 can be factored into distinct squarefree semiprimes in seven ways:

  (6*10*15*77)

  (6*10*21*55)

  (6*10*33*35)

  (6*14*15*55)

  (6*15*22*35)

  (10*14*15*33)

  (10*15*21*22),

so 69300 is in the sequence. A complete list of all strict factorizations of 24 is: (2*3*4), (2*12), (3*8), (4*6), (24), all of which contain at least one number that is not a squarefree semiprime, so 24 is not in the sequence.

MATHEMATICA

sqs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[sqs[n/d], Min@@#>d&]], {d, Select[Divisors[n], SquareFreeQ[#]&&PrimeOmega[#]==2&]}]];

Select[Range[100], sqs[#]!={}&]

CROSSREFS

A309356 is a kind of universal embedding.

A320894 is the complement in A028260.

A320911 lists all (not just distinct) products of squarefree semiprimes.

A339560 counts the partitions with these Heinz numbers.

A339661 has nonzero terms at these positions.

A001358 lists semiprimes, with squarefree case A006881.

A005117 lists squarefree numbers.

A320656 counts factorizations into squarefree semiprimes.

The following count vertex-degree partitions and give their Heinz numbers:

- A058696 counts partitions of 2n (A300061).

- A000070 counts non-multigraphical partitions of 2n (A339620).

- A209816 counts multigraphical partitions (A320924).

- A320921 counts connected graphical partitions (A320923).

- A339655 counts non-loop-graphical partitions of 2n (A339657).

- A339656 counts loop-graphical partitions (A339658).

- A339617 counts non-graphical partitions of 2n (A339618).

- A000569 counts graphical partitions (A320922).

The following count partitions of even length and give their Heinz numbers:

- A027187 has no additional conditions (A028260).

- A096373 cannot be partitioned into strict pairs (A320891).

- A338914 can be partitioned into strict pairs (A320911).

- A338915 cannot be partitioned into distinct pairs (A320892).

- A338916 can be partitioned into distinct pairs (A320912).

- A339559 cannot be partitioned into distinct strict pairs (A320894).

- A339560 can be partitioned into distinct strict pairs (A339561 [this sequence]).

Cf. A001055, A001221, A002100, A007717, A030229, A112798, A320655, A320893, A338899, A338903, A339563, A339659.

Sequence in context: A268390 A265693 A211484 * A346014 A006881 A030229

Adjacent sequences:  A339558 A339559 A339560 * A339562 A339563 A339564

KEYWORD

nonn

AUTHOR

Gus Wiseman, Dec 13 2020

STATUS

approved

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Last modified September 22 17:12 EDT 2021. Contains 347607 sequences. (Running on oeis4.)