12.07.2015 Views

Vol. 10 No 3 - Pi Mu Epsilon

Vol. 10 No 3 - Pi Mu Epsilon

Vol. 10 No 3 - Pi Mu Epsilon

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Rachele DernbowskiSUNY, Stony BrookLet I@) (where m is a positive integer) denote the number of partitionshaving m parts in which the kth part is less than or equal to m - k + 1 .1For example,/(I) = 1: 1l(2) = 2: 11,21l(3) = 5: 111, 211, 221, 311, 321l(4) = 14: 1111, 2111, 2211, 2221, 3111, 3211, 3221,3311,3321,4111,4211,4221,4311,4321.In this note we will show thatContinuing this process and using the values for Il(m) and 12(m) we getSince 13(m) = 2. we have, since the sum contains m - 3 Is,I3(m) = (m - 3) + [(m - 2) + (m - 3) + - + 21 + 2In a similar manner, starting with14(m) = ll(m - 1) + 12(m - 1) + 13(m - 1) + 14(m - 1)and using the formula for Ii(m), we getthe well-known Catalan numbers.Let Ik(m) denote the number of partitions counted in I(m) having largestpart k. For example,1](4) = 1 , li(4) = 3, 13(4) = 5, 14(4) = 5,We will get a formula for Ik(m) . Clearly, ll (m) = 1, the only partitionbeing 111 - 1 (m Is). For Ii(m) there are m - 1 places where therightmost 2 can be placed, so /->(m) = m - 1 . To count I3(m) note that weget a suitable partition by placing a 3 on the left of any partition into m - 1parts with largest part 3, so13(m) = Il(m - 1) + li(m - 1) + 13(m - 1)Applying this to the last term, we have13(m) = ll(m - 1) + b(m - 1) + ll(m - 2) + 12(m - 2) + 13(m - 2).Then, using mathematical induction, we can show that for t 2 3,Since f(m) = zrn I= I i,(m) , using the last formula and the identitywe find that, for m S 3,It is not hard to show that this is equivalent to

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