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URL: https://oeis.org/A300813

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A300813
a(n) is the smallest multiple of A001414(a(n-1)) not yet seen in the sequence; a(1)=2.
5
2, 4, 8, 6, 5, 10, 7, 14, 9, 12, 21, 20, 18, 16, 24, 27, 36, 30, 40, 11, 22, 13, 26, 15, 32, 50, 48, 33, 28, 44, 45, 55, 64, 60, 72, 84, 42, 96, 39, 80, 52, 17, 34, 19, 38, 63, 65, 54, 66, 112, 75, 78, 90, 91, 100, 56, 104, 57, 88, 51, 120, 70, 98, 128, 126, 105, 135, 140, 144, 154, 160, 150, 165, 76, 23
OFFSET
1,1
COMMENTS
If for some n, A001414(a(n-1)) is a prime p not seen before, then a(n)=p and a(n+1)=2*p. If the lesser of a twin prime pair is a(n), the greater is a(n+2).
The numbers not appearing in the first 10^7 terms are 1, 3, 37957, 37963, 38557, 39301, 40237, 40343, 40351, 40357, ...; it seems that all numbers other than 1 and 3 eventually appear. - Charles R Greathouse IV, Apr 09 2018
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..702396
EXAMPLE
a(2)=4 because A001414(2)=2 and 4 is the least multiple of 2 not yet seen.
a(3)=8 because A001414(4)=4 and 8 is the least multiple of 4 not seen yet.
MATHEMATICA
Nest[Append[#, Block[{k = 1, m = Total@ Flatten@ Map[ConstantArray[#1, #2] & @@ # &, FactorInteger@ Last@ #]}, While[! FreeQ[#, k m], k++]; k m]] &, {2}, 74] (* Michael De Vlieger, Mar 14 2018 *)
PROG
(PARI) sopfr(n, f=factor(n))=sum(i=1, #f[, 1], f[i, 1]*f[i, 2])
first(n)=my(v=vector(n), s=[2], t); v[1]=2; for(m=2, n, t=sopfr(v[m-1]); forstep(k=t, t*m, t, if(setsearch(s, k), next); s=setunion(s, [k]); v[m]=k; break)); v \\ Charles R Greathouse IV, Apr 06 2018
CROSSREFS
Cf. A001414.
Sequence in context: A354727 A329455 A292808 * A086354 A014257 A247576
KEYWORD
nonn,look
AUTHOR
STATUS
approved