OFFSET
1,4
LINKS
Bence Bernáth, Table of n, a(n) for n = 1..10000
EXAMPLE
For n = 4, a(4) = 2, which appeared only once in the sequence so a(5)=1-2=-1.
MATHEMATICA
a = {0}; Do[AppendTo[a, Count[a, a[[-1]]] - a[[-1]]], {100}]; a (* Amiram Eldar, Nov 18 2020 *)
a[1] = 0; a[n_Integer?Positive] := a[n] = Count[Array[a, n - 1], a[n - 1]] - a[n - 1]; Array[a, 101] (* Jan Mangaldan, Nov 27 2020 *)
PROG
(MATLAB)
length=1000;
sequence(1)=0;
for n=2:1:length
sequence(n)=nnz(sequence==sequence(end))-sequence(n-1);
end
(PARI) lista(nn) = {my(va = vector(nn)); for (n=2, nn, va[n] = #select(x->(x==va[n-1]), vector(n-1, k, va[k])) - va[n-1]; ); va; } \\ Michel Marcus, Nov 18 2020
(Python)
from itertools import islice
from collections import Counter
def agen(): # generator of terms
an, k = 0, Counter()
while True:
yield an
k[an] += 1
an = k[an] - an
print(list(islice(agen(), 86))) # Michael S. Branicky, Nov 12 2022
CROSSREFS
KEYWORD
sign,easy
AUTHOR
Bence Bernáth, Nov 17 2020
STATUS
approved