|
|
A181898
|
|
Smallest positive integer which cannot be calculated by an expression containing n binary operators (any of add, subtract, multiply and divide) whose operands are any integer between 1 and 9; parentheses allowed.
|
|
8
|
|
|
|
OFFSET
|
0,1
|
|
LINKS
|
|
|
EXAMPLE
|
a(2)=92 because at least 3 operators are required, e.g., (2*7 + 9)*4.
|
|
PROG
|
(R) See Jones link.
(PARI) first(n)=my(op=[(x, y)->x+y, (x, y)->x-y, (x, y)->y-x, (x, y)->x*y, (x, y)->x/y, (x, y)->y/x], v=vector(n+1), t); v[1]=[1..9]; for(k=2, #v, my(u=[]); for(i=1, (k+1)\2, my(a=v[i], b=v[k-i]); t=Set(concat(apply(f->setbinop(f, a, b), op))); u=concat(u, t)); v[k]=setminus(Set(u), [0])); t=10; for(i=1, #v, while(setsearch(v[i], t), t++); v[i]=t); v \\ Charles R Greathouse IV, Jan 09 2017
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|