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A303762
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a(0) = 1, and for n >= 1, a(n) is either the largest divisor of a(n-1) not already present in the sequence, or (if all divisors already used), a(n-1) * {the least prime p such that p does not divide a(n-1) and p*a(n-1) is not already present}.
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7
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1, 2, 6, 3, 15, 5, 10, 30, 210, 105, 35, 7, 14, 42, 21, 231, 77, 11, 22, 66, 33, 165, 55, 110, 330, 2310, 1155, 385, 770, 154, 462, 6006, 3003, 1001, 143, 13, 26, 78, 39, 195, 65, 130, 390, 2730, 1365, 455, 91, 182, 546, 273, 4641, 1547, 221, 17, 34, 102, 51, 255, 85, 170, 510, 3570, 1785, 595, 119, 238, 714, 357, 3927, 1309, 187, 374, 1122, 561, 2805, 935
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OFFSET
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0,2
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COMMENTS
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Each a(n+1) is either a divisor or a multiple of a(n).
The construction is otherwise like that of A303760, except here we choose the largest divisor instead of the smallest one. In contrast to A303760, this sequence is NOT permutation of A005117: 70 = A019565(13) is the first missing squarefree number. See also comments in A303769, A303749 and A302775.
Index of greatest prime factor of a(n) is monotonic and increments at n = {0, 1, 2, 4, 8, 15, 31, 50, 102, 157, 317, 480, 964, 1451, 2907, 4366, 8738, 13113, 26233, 39356, ...} - Michael De Vlieger, May 22 2018
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LINKS
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FORMULA
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EXAMPLE
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Table below shows the initial 31 terms at right. First column is index n. Second shows "." if a(n) = largest divisor of a(n-1), or factor p. Third shows presence "1" or absence "." of prime k among prime divisors of a(n).
n p\d MN(n) a(n)
---------------------------
0 . . 1
1 2 1 2
2 3 11 6
3 . .1 3
4 5 .11 15
5 . ..1 5
6 2 1.1 10
7 3 111 30
8 7 1111 210
9 . .111 105
10 . ..11 35
11 . ...1 7
12 2 1..1 14
13 3 11.1 42
14 . .1.1 21
15 11 .1.11 231
16 . ...11 77
17 . ....1 11
18 2 1...1 22
19 3 11..1 66
20 . .1..1 33
21 5 .11.1 165
22 . ..1.1 55
23 2 1.1.1 110
24 3 111.1 330
25 7 11111 2310
26 . .1111 1155
27 . ..111 385
28 2 1.111 770
29 . 1..11 154
30 3 11.11 462
31 13 11.111 6006
... (End)
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MATHEMATICA
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Nest[Append[#, Block[{d = Divisors@ #[[-1]], p = 2}, If[Complement[d, #] != {}, Complement[d, #][[-1]], While[Nand[Mod[#[[-1]], p] != 0, FreeQ[#, p #[[-1]] ] ], p = NextPrime@ p]; p #[[-1]] ] ] ] &, {1}, 75] (* Michael De Vlieger, May 22 2018 *)
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PROG
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(PARI)
default(parisizemax, 2^31);
up_to = 2^14;
v303762 = vector(up_to);
m_inverses = Map();
prev=1; for(n=1, up_to, fordiv(prev, d, if(!mapisdefined(m_inverses, (prev/d)), v303762[n] = (prev/d); mapput(m_inverses, (prev/d), n); break)); if(!v303762[n], apu = prev; while(mapisdefined(m_inverses, try = prev*A053669(apu)), apu *= A053669(apu)); v303762[n] = try; mapput(m_inverses, try, n)); prev = v303762[n]);
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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