A338905 - OEIS

A338905

Irregular triangle read by rows where row n lists all squarefree semiprimes with prime indices summing to n.

6, 10, 14, 15, 21, 22, 26, 33, 35, 34, 39, 55, 38, 51, 65, 77, 46, 57, 85, 91, 58, 69, 95, 119, 143, 62, 87, 115, 133, 187, 74, 93, 145, 161, 209, 221, 82, 111, 155, 203, 247, 253, 86, 123, 185, 217, 299, 319, 323, 94, 129, 205, 259, 341, 377, 391, 106, 141

(list; graph; refs; listen; history; text; internal format)

OFFSET

3,1

COMMENTS

A squarefree semiprime is a product of any two distinct prime numbers. A prime index of n is a number m such that the m-th prime number divides n. The multiset of prime indices of n is row n of A112798.

LINKS

Table of n, a(n) for n=3..60.

EXAMPLE

Triangle begins:

14 15

21 22

26 33 35

34 39 55

38 51 65 77

46 57 85 91

58 69 95 119 143

62 87 115 133 187

74 93 145 161 209 221

82 111 155 203 247 253

86 123 185 217 299 319 323

MATHEMATICA

Table[Sort[Table[Prime[k]*Prime[n-k], {k, (n-1)/2}]], {n, 3, 10}]

CROSSREFS

A004526 (shifted right) gives row lengths.

A025129 (shifted right) gives row sums.

A056239 gives sum of prime indices (Heinz weight).

A339116 is a different triangle whose diagonals are these rows.

A338904 is the not necessarily squarefree version, with row sums A024697.

A338907/A338908 are the union of odd/even rows.

A339114/A332765 are the row minima/maxima.

A001358 lists semiprimes, with odd/even terms A046315/A100484.

A005117 lists squarefree numbers.

A006881 lists squarefree semiprimes, with odd/even terms A046388/A100484.

A087112 groups semiprimes by greater factor.

A168472 gives partial sums of squarefree semiprimes.

A338898, A338912, and A338913 give the prime indices of semiprimes, with product A087794, sum A176504, and difference A176506.

A338899, A270650, and A270652 give the prime indices of squarefree semiprimes, with difference A338900.

Cf. A000040, A001221, A014342, A098350, A112798, A320656, A338901, A338906, A339003, A339004, A339005, A339115.

Sequence in context: A201464 A162730 A180074 * A093772 A046400 A100660

Adjacent sequences: A338902 A338903 A338904 * A338906 A338907 A338908

KEYWORD

nonn,tabf

AUTHOR

Gus Wiseman, Nov 28 2020

STATUS

approved