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A240588 a(1) = 1, a(2) = 2; for n >= 3, a(n) = least number not included earlier that divides the concatenation of all previous terms. 11
1, 2, 3, 41, 7, 9, 137131, 61, 2023244487101, 13, 19, 11, 143, 142733, 21, 17, 193, 37, 3907, 1290366811360047359, 1805030483980039, 3803623, 123, 369, 27, 23, 58271, 47609, 523, 79, 307, 179, 73, 57, 18032419296851, 29, 31, 3281881401611107, 69, 171, 60244474373, 197, 97 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
From Scott R. Shannon, Dec 19 2019: (Start)
The next unknown term a(131) requires the factorization of a 517-digit composite number 46297...2963. (End)
LINKS
EXAMPLE
a(1)=1 and a(2)=2. a(1) U a(2) = 12 and its divisors are 1, 2, 3, 4, 6, 12. Therefore 3 is the least number not yet present in the sequence which divides 12. Again, a(1) U a(2) U a(3) = 123 and its divisors are 1, 3, 41, 123. Therefore a(4)=41. Etc.
MAPLE
with(numtheory):
T:=proc(t) local x, y; x:=t; y:=0; while x>0 do x:=trunc(x/10); y:=y+1; od; end:
P:=proc(q) local a, b, c, k, n; b:=12; print(1); print(2); c:=[1, 2];
for n from 1 to q do a:=sort([op(divisors(b))]); for k from 2 to nops(a) do
if not member(a[k], c) then c:=[op(c), a[k]]; b:=a[k]+b*10^T(a[k]); print(a[k]); break;
fi; od; od; end: P(19);
MATHEMATICA
a = {1, 2}; While[Length[a] < 22,
n = ToExpression[StringJoin[ToString /@ a]];
AppendTo[a, SelectFirst[Sort[Divisors[n]], FreeQ[a, #] &]]
]; a
CROSSREFS
Sequence in context: A242174 A340394 A288519 * A013646 A059800 A330293
KEYWORD
nonn,base
AUTHOR
Paolo P. Lava, Apr 29 2014
EXTENSIONS
a(20)-a(40) from Alois P. Heinz, May 08 2014
a(22) corrected by Ryan Hitchman, Sep 14 2017
a(23)-a(25) from Robert Price, May 16 2019
a(23)-a(25) corrected, and a(26)-a(43) added by Scott R. Shannon, Dec 10 2019
STATUS
approved

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Last modified March 29 04:23 EDT 2024. Contains 371264 sequences. (Running on oeis4.)