

A359007


a(n) = b(nb(n)) where b is Van Eck's sequence A181391.


0



0, 0, 0, 0, 1, 0, 2, 0, 2, 0, 0, 2, 0, 5, 1, 0, 0, 0, 2, 4, 0, 5, 0, 6, 0, 3, 0, 0, 2, 5, 0, 4, 14, 6, 3, 0, 6, 15, 5, 3, 9, 0, 5, 3, 0, 6, 5, 0, 3, 8, 3, 6, 0, 3, 2, 0, 0, 5, 9, 0, 4, 1, 0, 0, 3, 32, 0, 4, 11, 0, 7, 17, 0, 3, 11, 0, 2, 31, 6, 31, 0, 0, 6, 3, 0, 9, 2, 33, 3, 0, 3, 15, 0, 5
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OFFSET

1,7


COMMENTS

In Van Eck's sequence, b(n) is the distance between b(n1) and the previous occurrence of b(n1) there. Taking a(n) = b(nb(n)) is therefore the distance between the second and third last occurrence of b(n1) there.
If b(n1) has not yet occurred three times then the result is a(n) = 0 either by b(n)=0 when b(n1) has only occurred once, or b(nb(n)) = 0 when b(n1) has only occurred twice.


LINKS



EXAMPLE

b(14) is 2, so we count back two steps to b(12), which is 5. Therefore a(14) = 5. As b(141) = b(13) = 0, the three occurrences of 0's are separated by b(14) and b(12) = a(14), that is, 2 and 5 steps:
.
n: ... 5 6 7 8 9 10 11 12 13 14 15 16 ...
.
<5><2>
  
  
b(n): ... 2, 0, 2, 2, 1, 6, 0, 5, 0, 2, 6, 5, ...
 
v 
a(14) = 5 <
two steps
back


PROG

(PARI) A181391_vec(N, a=0, i=Map())={vector(N, n, a=if(n>1, iferr(nmapget(i, a), E, 0)+mapput(i, a, n)))};


CROSSREFS



KEYWORD

nonn,easy


AUTHOR



STATUS

approved



