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A337486

Numbers k such that b(k+1) divides b(k), where b() is Recamán's multiplicative sequence A008336.

1, 6, 10, 12, 14, 18, 20, 22, 26, 28, 30, 34, 36, 38, 42, 44, 45, 46, 50, 52, 54, 58, 60, 66, 68, 70, 72, 78, 82, 84, 86, 90, 92, 93, 94, 95, 98, 99, 100, 104, 106, 110, 111, 114, 116, 118, 119, 122, 124, 126, 130, 132, 134, 135, 136, 142, 146, 147, 148, 150, 154, 156, 158, 161, 162, 164, 165, 166

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OFFSET

1,2

COMMENTS

The old definition was: 1, together with the numbers formed by removing the required prime factors to form the number from a set which is initially empty and that has primes added via the addition of the prime factors of numbers which cannot be created from those currently in the set. Start by trying to create the number 2.

Consider an initially empty set of primes whose numbers are used to create a given number where each time a number is created those prime factors are removed from the set. If a number cannot be created as all its required prime factors are not currently in the set then all the prime factors of that number are instead added to the set. Start by trying to create the number 2 followed by all other integers. This sequence list the numbers that are created.

For the first 1 million terms the largest gap between terms is 15, between a(88018) = 189648 and a(88019) = 189663. The 1 millionth term is created after the addition of the number 2123404. The prime set at that point has 114323 primes, with the maximum number of entries for a single prime being eleven, for 887.

Note that if after the creation of a number the entire set of primes is cleared then the numbers created are those in A109895.

LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..10000

EXAMPLE

a(2) = 6. As there are no primes initially in the set 2,3,4,5 cannot be created and instead these numbers add three 2's, one 3 and one 5 to the set. As there is now one 2 and one 3 the number 6 = 2*3 can be created. After 6 is created the set of primes now contains 2,2,5.

a(3) = 10. After 7,8,9, none of which can be created from the prime set, the prime set contains 2,2,2,2,2,3,3,5,7. As 2 and 5 are present 10 = 2*5 can be created, after which the set contains 2,2,2,2,3,3,7.

MATHEMATICA

Block[{nn = 166, k = 1}, Reap[Do[If[Mod[k, i] == 0, k /= i; Sow[i], k *= i], {i, nn}]][[-1, 1]]] (* Michael De Vlieger, Sep 12 2020 *)

PROG

(Python)

from itertools import count, islice

def A337486_gen(): # generator of terms

c = 1

for n in count(1):

a, b = divmod(c, n)

if not b:

c = a

yield n