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A147980 Given a set of positive integers A={1,2,...,n-1,n}, n>=2. Take subsets of A of the form {1,...,n} so only subsets containing numbers 1 and n are allowed. Then a(1)=1 and a(n) is the number of subsets where arithmetic mean of the subset is an integer. 1
1, 0, 2, 0, 4, 4, 8, 12, 28, 44, 84, 156, 288, 540, 1020, 1904, 3616, 6860, 13024, 24836, 47448, 90772, 174072, 334348, 643112, 1238928, 2389956, 4615916, 8925808, 17278680, 33482196, 64944060, 126083448, 244989096, 476416560, 927167752, 1805691728, 3519062820 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

For n odd the value of the arithmetic mean for each possible subset equals (n+1)/2. For n even this value is n/2 or (n+2)/2. If looking after RootMeanSquare for the subset we obtain a sequence [1,0,0,0,0,0,2,...]. We see for example for n=7, A={1,2,3,4,5,6,7} and the only 2 subsets with an integer RootMeanSquare are {1,7}, {1,5,7}. Interestingly the value of RootMeanSquare is 5 for both subsets. So the sequence A140480 RMS numbers is a subsequence of it as a set of divisors of n is clearly a subset of n of the form {1,...,n}.

LINKS

Alois P. Heinz, Table of n, a(n) for n = 1..100

Eric Weisstein's World of Mathematics, Arithmetic mean

EXAMPLE

n=5, A={1,2,3,4,5}. Subsets of A starting with 1 and ending with 5 are : {1,5}, {1,2,5}, {1,3,5}, {1,4,5}, {1,2,3,5}, {1,2,4,5}, {1,3,4,5}, {1,2,3,4,5}. Arithmetic mean of the subset is an integer for subsets : {1,5}, {1,3,5}, {1,2,4,5}, {1,2,3,4,5}. Thus a(5) = 4. The value of the arithmetic mean is 3 for all 4 subsets.

MAPLE

b:= proc(i, s, c) option remember; `if` (i=1, `if` (irem (s, c)=0, 1, 0), b(i-1, s, c)+ b(i-1, s+i, c+1)) end: a:= n-> `if` (n=1, 1, b (n-1, n+1, 2)): seq (a(n), n=1..40);  # Alois P. Heinz, May 06 2010

CROSSREFS

Cf. A140480.

Sequence in context: A145811 A131772 A304877 * A021493 A195395 A296805

Adjacent sequences:  A147977 A147978 A147979 * A147981 A147982 A147983

KEYWORD

nonn

AUTHOR

Ctibor O. Zizka, Nov 18 2008

EXTENSIONS

More terms from Alois P. Heinz, May 06 2010

STATUS

approved

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Last modified March 18 12:10 EDT 2019. Contains 321283 sequences. (Running on oeis4.)