

A140606


Number of inequivalent expressions involving n operands.


2



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 ac+b and a+bc but (a+b)c is different from ab+c.
Another example is expression (ab)/(cd) is equivalent to (ba)/(dc).


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;ab;ba;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



