A348086 - OEIS

A348086

Variation of the EKG sequence A064413: a(n) is the smallest number not already used which shares a factor with a(n-1) and has a different number of divisors than a(n-1).

1, 2, 4, 6, 3, 9, 12, 8, 16, 10, 5, 15, 18, 14, 7, 21, 24, 20, 22, 11, 33, 30, 25, 35, 28, 26, 13, 39, 36, 27, 42, 32, 34, 17, 51, 45, 40, 38, 19, 57, 48, 44, 46, 23, 69, 54, 50, 55, 60, 52, 56, 49, 63, 66, 58, 29, 87, 72, 62, 31, 93, 75, 65, 70, 64, 68, 74, 37, 111, 78, 76, 80, 82, 41, 123, 81

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OFFSET

1,2

COMMENTS

This sequence uses the same rules as A064413 except with the additional restriction that a(n) must have a different number of divisors than a(n-1). Despite this additional restriction the overall behavior of the sequence is almost identical to A064413, with most values appearing only a few places away from their position in A064413.

The fixed points below 10000 are 1, 2, 8, 10, 14, 26, 32, 38, 259. It is likely no more exist.

LINKS

Table of n, a(n) for n=1..76.

EXAMPLE

a(9) = 16 as a(8) = 8 which shares a factor with 16, 16 is unused, and tau(8) = 4 does not equal tau(16) = 5. This is the first term that differs from A064413, the later having a(9) = 10, but tau(10) = 4 = tau(8) so is forbidden here.

MATHEMATICA

a[1]=1; a[2]=2; a[n_]:=a[n]=(k=1; While[GCD[k, a[n-1]]==1||SameQ@@DivisorSigma[0, {k, a[n-1]}]||MemberQ[Array[a, n-1], k], k++]; k); Array[a, 76] (* Giorgos Kalogeropoulos, Oct 08 2021 *)

CROSSREFS

Cf. A064413, A000005, A347984, A098550, A336957.

Sequence in context: A342366 A336946 A357777 * A122280 A365259 A356851

Adjacent sequences: A348083 A348084 A348085 * A348087 A348088 A348089

KEYWORD

nonn

AUTHOR

Scott R. Shannon, Sep 28 2021

STATUS

approved