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Search: id:A102781
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| A102781 |
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Integer part of n#/(n-2)#/2#. |
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+0
3
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| 0, 1, 2, 3, 5, 6, 8, 9, 11, 14, 15, 18, 20, 21, 23, 26, 29, 30, 33, 35, 36, 39, 41, 44, 48, 50, 51, 53, 54, 56, 63, 65, 68, 69, 74, 75, 78, 81, 83, 86, 89, 90, 95, 96, 98, 99, 105, 111, 113, 114, 116, 119, 120, 125, 128, 131, 134, 135, 138, 140, 141, 146
(list; graph; listen)
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OFFSET
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2,3
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COMMENT
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0# = 1# = 2 by convention.
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FORMULA
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n# = product of primes <= n. 0#=1#=2. n#/(n-r)#/r# is analogous to the number of combinations of n things taken r at a time: C(n, r) = n!/(n-r)!/r! where factorial ! is replaced by primorial #.
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PROGRAM
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(PARI) c(n, r) = { local(p); forprime(p=r, n, print1(floor(primorial(p)/primorial(p-r)/primorial(r)+.0)", ") ) } primorial(n) = \ The product primes <= n using the pari primelimit. { local(p1, x); if(n==0||n==1, return(2)); p1=1; forprime(x=2, n, p1*=x); return(p1) }
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CROSSREFS
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Sequence in context: A097702 A082583 A130290 this_sequence A005097 A111332 A139791
Adjacent sequences: A102778 A102779 A102780 this_sequence A102782 A102783 A102784
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KEYWORD
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easy,nonn
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AUTHOR
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Cino Hilliard (hillcino368(AT)gmail.com), Feb 25 2005
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