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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 (list; graph; refs; listen; history; text; internal format)
 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] LINKS 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 A009882 Adjacent sequences:  A145018 A145019 A145020 * A145022 A145023 A145024 KEYWORD easy,nonn AUTHOR Ken Levasseur, Sep 29 2008 STATUS approved

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Last modified January 19 22:09 EST 2021. Contains 340298 sequences. (Running on oeis4.)