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A003687
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a(n+1)=a(n)-a(1)a(2)...a(n-1), if n>0. a(0)=1,a(1)=2.
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3
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1, 2, 1, -1, -3, -1, -7, -1, -43, -1, -1807, -1, -3263443, -1, -10650056950807, -1, -113423713055421844361000443, -1, -12864938683278671740537145998360961546653259485195807, -1
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OFFSET
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0,2
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COMMENTS
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a(n)=a(n-1)-a(n-2)^2+a(n-1)*a(n-2), if n>2. - Michael Somos Mar 19 2004
Consider the mapping f(a/b) = (a - b)/(ab). Taking a = 2 b = 1 to start with and carrying out this mapping repeatedly on each new (reduced) rational number gives the following sequence 2/1,1/2,-1/2,-3/-2,-1/6,... Sequence contains the numerators. - Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Mar 24 2003
An infinite coprime sequence defined by recursion. - Michael Somos Mar 19 2004
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LINKS
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Table of n, a(n) for n=0..19.
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PROG
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(PARI) a(n)=local(an); if(n<1, (n==0), an=vector(max(2, n)); an[1]=2; an[2]=1; for(k=3, n, an[k]=an[k-1]-an[k-2]^2+an[k-1]*an[k-2]); an[n])
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CROSSREFS
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Cf. A081478.
For n>1, a(2n-1) = -1, a(2n) = -A007018(n-1) - 1.
Sequence in context: A052920 A089141 A170820 * A104575 A173937 A046223
Adjacent sequences: A003684 A003685 A003686 * A003688 A003689 A003690
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KEYWORD
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sign,easy
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AUTHOR
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N. J. A. Sloane.
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STATUS
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approved
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