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A123144
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a(1) = 1, a(n) = a(n-1) if n = 1 (mod 3), otherwise n*a(n-1).
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2
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1, 2, 6, 6, 30, 180, 180, 1440, 12960, 12960, 142560, 1710720, 1710720, 23950080, 359251200, 359251200, 6107270400, 109930867200, 109930867200, 2198617344000, 46170964224000, 46170964224000, 1061932177152000, 25486372251648000, 25486372251648000
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OFFSET
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1,2
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COMMENTS
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Without duplicates this sequence is A293653.
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LINKS
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MAPLE
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a:= proc(n) option remember; `if`(n=0, 1,
`if`(irem(n, 3)=1, 1, n)*a(n-1))
end:
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MATHEMATICA
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a[n_]:= a[n]= If[n==1, 1, If[Mod[n, 3]==1, a[n-1], n*a[n-1]]];
Table[a[n], {n, 30}]
nxt[{n_, a_}]:={n+1, If[Mod[n+1, 3]==1, a, a(n+1)]}; Transpose[NestList[nxt, {1, 1}, 20]][[2]] (* Harvey P. Dale, Jul 22 2014 *)
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PROG
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(Magma)
function a(n)
if n eq 1 then return 1;
elif (n mod 3) eq 1 then return a(n-1);
else return n*a(n-1);
end if;
end function;
(SageMath)
if (n==1): return 1
elif (n%3==1): return a(n-1)
else: return n*a(n-1)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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