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A145021
a(n) = number of different positive integers that can be formed from different groupings of expressions of the form n op1 n op2 n op3 n, where each of op1, op2 and op3 are addition, subtraction, multiplication or division.
0
4, 10, 20, 25, 27, 29, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30
OFFSET
1,1
COMMENTS
If one uses all 4^3=64 forms of this type but no parentheses, the sequence starts 4,9,15,13,15,14... In this case 4/4/4/4=1/4/4=1/16 is not an integer (association left-to-right), whereas with parenthesis one could write (4/4)/(4/4)=1, an integer, for example. The definition need clarification in this respect. [From R. J. Mathar, Jan 22 2009]
FORMULA
If k >3, a(2k-1)=30 and a(2k)=31. - Ken Levasseur, Oct 01 2008
EXAMPLE
You can form the numbers 1, 2, 3, 4 with 4 ones; hence the first term is 4.
CROSSREFS
Sequence in context: A268221 A086176 A015789 * A135280 A100436 A348011
KEYWORD
easy,nonn
AUTHOR
Ken Levasseur, Sep 29 2008
STATUS
approved