A363729 Numbers that are not a power of a prime but whose prime indices satisfy (mean) = (median) = (mode), assuming there is a unique mode.

90, 270, 525, 550, 756, 810, 1666, 1911, 1950, 2268, 2430, 2625, 2695, 2700, 2750, 5566, 6762, 6804, 6897, 7128, 7290, 8100, 8500, 9310, 9750, 10285, 10478, 11011, 11550, 11662, 12250, 12375, 12495, 13125, 13377, 13750, 14014, 14703, 18865, 19435, 20412, 21384

Offset

1,1

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.

A mode in a multiset is an element that appears at least as many times as each of the others. For example, the modes in {a,a,b,b,b,c,d,d,d} are {b,d}.

The median of a multiset is either the middle part (for odd length), or the average of the two middle parts (for even length).

Links

Table of n, a(n) for n=1..42.

Example

The prime indices of 6897 are {2,5,5,8}, with mean 5, median 5, and modes {5}, so 6897 is in the sequence.
The terms together with their prime indices begin:
     90: {1,2,2,3}
    270: {1,2,2,2,3}
    525: {2,3,3,4}
    550: {1,3,3,5}
    756: {1,1,2,2,2,4}
    810: {1,2,2,2,2,3}
   1666: {1,4,4,7}
   1911: {2,4,4,6}
   1950: {1,2,3,3,6}
   2268: {1,1,2,2,2,2,4}
   2430: {1,2,2,2,2,2,3}

Mathematica

prix[n_]:=If[n==1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];
modes[ms_]:=Select[Union[ms], Count[ms, #]>=Max@@Length/@Split[ms]&];
Select[Range[1000], !PrimePowerQ[#]&&{Mean[prix[#]]}=={Median[prix[#]]}==modes[prix[#]]&]

Crossrefs

For just primes instead of prime powers we have A363722.

Including prime-powers gives A363727, counted by A363719.

These partitions are counted by A363728.

For unequal instead of equal we have A363730, counted by A363720.

A000961 lists the prime powers, complement A024619.

A112798 lists prime indices, length A001222, sum A056239.

A326567/A326568 gives mean of prime indices.

A356862 ranks partitions with a unique mode, counted by A362608.

A359178 ranks partitions with multiple modes, counted by A362610.

A360005 gives twice the median of prime indices.

A362611 counts modes in prime indices, triangle A362614.

A362613 counts co-modes in prime indices, triangle A362615.

A363486 gives least mode in prime indices, A363487 greatest.

Just two statistics:

- (mean) = (median): A359889, counted by A240219.

- (mean) != (median): A359890, counted by A359894.

- (mean) = (mode): counted by A363723, see A363724, A363731.

- (median) = (mode): counted by A363740.

Cf. A215366, A327473, A327476, A359893, A359908, A360009, A360248, A360550, A363721, A363725, A363741.

Sequence in context: A074213 A231961 A237131 * A255784 A250877 A250878

Adjacent sequences: A363726 A363727 A363728 * A363730 A363731 A363732

Keyword

nonn

Author

Gus Wiseman, Jun 24 2023

Status

approved