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A140606 Number of inequivalent expressions involving n operands. 1
1, 6, 68, 1170, 27142, 793002, 27914126, 1150212810, 54326011414, 2894532443154, 171800282010062, 11243812043430330, 804596872359480358, 62506696942427106498, 5239819196582605428254, 471480120474696200252970, 45328694990444455796547766, 4637556923393331549190920306 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Only operators +, -, *, / (and with parentheses) are permitted.

Expression (a+b)-c is equivalent to a-c+b and a+b-c but (a+b)-c is different from a-b+c.

Another example is expression (a-b)/(c-d) is equivalent to (b-a)/(d-c).

LINKS

Table of n, a(n) for n=1..18.

Author?, A Chinese web page containing the first 100 terms and C source code to generate it

Author?, A Chinese web page where the problem originated

EXAMPLE

When n=2, there are six inequivalent expressions: a+b;a-b;b-a;a*b;a/b;b/a while other expression such as b+a is equivalent to a+b

CROSSREFS

Sequence in context: A006737 A128869 A186669 * A014505 A127184 A113692

Adjacent sequences:  A140603 A140604 A140605 * A140607 A140608 A140609

KEYWORD

nonn,nice

AUTHOR

Du, Zhao Hui, Jul 07 2008

STATUS

approved

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Last modified April 23 06:16 EDT 2014. Contains 240913 sequences.