A322846 - OEIS

A322846

Squarefree numbers whose prime indices have no equivalent primes.

1, 2, 3, 5, 6, 7, 10, 11, 14, 15, 17, 19, 21, 22, 23, 30, 31, 33, 34, 35, 37, 38, 39, 41, 42, 46, 51, 53, 55, 57, 59, 61, 62, 65, 66, 67, 69, 70, 71, 74, 77, 78, 82, 83, 85, 87, 89, 91, 93, 95, 97, 102, 103, 105, 106, 107, 109, 110, 111, 114, 115, 118, 119

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OFFSET

1,2

COMMENTS

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.

In an integer partition, two primes are equivalent if each part has in its prime factorization the same multiplicity of both primes. For example, in (6,5) the primes {2,3} are equivalent while {2,5} and {3,5} are not. In (30,6) also, the primes {2,3} are equivalent, while {2,5} and {3,5} are not.

Also MM-numbers of strict T_0 multiset multisystems. A multiset multisystem is a finite multiset of finite multisets. The multiset multisystem with MM-number n is formed by taking the multiset of prime indices of each part of the multiset of prime indices of n. For example, the prime indices of 78 are {1,2,6}, so the multiset multisystem with MM-number 78 is {{},{1},{1,2}}. The dual of a multiset multisystem has, for each vertex, one block consisting of the indices (or positions) of the parts containing that vertex, counted with multiplicity. For example, the dual of {{1,2},{2,2}} is {{1},{1,2,2}}. The T_0 condition means the dual is strict (no repeated parts).

LINKS

Table of n, a(n) for n=1..63.

EXAMPLE

The sequence of all strict T_0 multiset multisystems together with their MM-numbers begins:

1: {}

2: {{}}

3: {{1}}

5: {{2}}

6: {{},{1}}

7: {{1,1}}

10: {{},{2}}

11: {{3}}

14: {{},{1,1}}

15: {{1},{2}}

17: {{4}}

19: {{1,1,1}}

21: {{1},{1,1}}

22: {{},{3}}

23: {{2,2}}

30: {{},{1},{2}}

31: {{5}}

33: {{1},{3}}

34: {{},{4}}

35: {{2},{1,1}}

37: {{1,1,2}}

38: {{},{1,1,1}}

39: {{1},{1,2}}

MATHEMATICA

primeMS[n_]:=If[n==1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];

dual[eds_]:=Table[First/@Position[eds, x], {x, Union@@eds}];

Select[Range[100], And[SquareFreeQ[#], UnsameQ@@dual[primeMS/@primeMS[#]]]&]

CROSSREFS

Cf. A000009, A005117, A056239, A059201, A112798, A302242, A302505, A316978, A316979, A316983, A319558, A319564, A319728, A322847.

Sequence in context: A137313 A246867 A028805 * A302496 A117344 A117204

Adjacent sequences: A322843 A322844 A322845 * A322847 A322848 A322849

KEYWORD

nonn

AUTHOR

Gus Wiseman, Dec 28 2018

STATUS

approved