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A335463
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Numbers k such that there exists a permutation of the prime indices of k matching both (1,2,1) and (2,1,2).
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20
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36, 72, 90, 100, 108, 126, 144, 180, 196, 198, 200, 216, 225, 234, 252, 270, 288, 300, 306, 324, 342, 350, 360, 378, 392, 396, 400, 414, 432, 441, 450, 468, 484, 500, 504, 522, 525, 540, 550, 558, 576, 588, 594, 600, 612, 630, 648, 650, 666, 675, 676, 684, 700
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OFFSET
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1,1
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COMMENTS
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A prime index of k is a number m such that prime(m) divides k. The multiset of prime indices of k is row k of A112798.
We define a pattern to be a finite sequence covering an initial interval of positive integers. Patterns are counted by A000670 and ranked by A333217. A sequence S is said to match a pattern P if there is a not necessarily contiguous subsequence of S whose parts have the same relative order as P. For example, (3,1,1,3) matches (1,1,2), (2,1,1), and (2,1,2), but avoids (1,2,1), (1,2,2), and (2,2,1).
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LINKS
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EXAMPLE
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The sequence of terms together with their prime indices begins:
36: {1,1,2,2}
72: {1,1,1,2,2}
90: {1,2,2,3}
100: {1,1,3,3}
108: {1,1,2,2,2}
126: {1,2,2,4}
144: {1,1,1,1,2,2}
180: {1,1,2,2,3}
196: {1,1,4,4}
198: {1,2,2,5}
200: {1,1,1,3,3}
216: {1,1,1,2,2,2}
225: {2,2,3,3}
234: {1,2,2,6}
252: {1,1,2,2,4}
270: {1,2,2,2,3}
288: {1,1,1,1,1,2,2}
300: {1,1,2,3,3}
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MATHEMATICA
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primeMS[n_]:=If[n==1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];
Select[Range[100], Select[Permutations[primeMS[#]], MatchQ[#, {___, x_, ___, y_, ___, x_, ___}/; x<y]&&MatchQ[#, {___, x_, ___, y_, ___, x_, ___}/; x>y]&]!={}&]
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CROSSREFS
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Replacing "and" with "or" gives A126706.
Positions of nonzero terms in A335462.
Permutations of prime indices are counted by A008480.
Unsorted prime signature is A124010. Sorted prime signature is A118914.
STC-numbers of permutations of prime indices are A333221.
Patterns matched by standard compositions are counted by A335454.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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