This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
A002487(33) = 6 and this is the first time 6 appears, so a(6) = 33.
aa = {}; a[0] = 0; a[1] = 1; a[n_] := a[n] = If[EvenQ[n], a[n/2], a[(n - 1)/2] + a[(n + 1)/2]]; Do[k = 0; While[a[k] != p, k++]; AppendTo[aa, k], {p, 0, 100}]; aa (* Artur Jasinski, Dec 06 2010 *)
fusc(n)={my(a=1, b=0);while(n,if(bitand(n, 1), b+=a, a+=b);n>>=1); b};
list(N)={
my(v=vector(N),k);
forstep(n=1,9e99,2,
k=fusc(n);
if(k<=N && !v[k],
v[k]=n;
if(vecmin(v),return(v))
)
)
}; \\ Charles R Greathouse IV, Dec 20 2011
from itertools import count from functools import reduce def A020946(n): return next(filter(lambda k:sum(reduce(lambda x,y:(x[0],x[0]+x[1]) if int(y) else (x[0]+x[1],x[1]),bin(k)[-1:2:-1],(1,0)))==n,count(1))) if n else 0 # Chai Wah Wu, May 05 2023
a[n_] := a[n] = Which[n < 2, Boole[n == 1] - Boole[n == 0], OddQ[n], Abs[a[n - 1] - a[n - 2]], True, a[n/2] + a[n/2 - 1]]; s = Array[a[#] &, 4200]; Array[FirstPosition[s, #][[1]] &, LengthWhile[Differences@ Union@ s, # == 1 &]] (* Michael De Vlieger, Feb 18 2022, after Michael Somos at A020944 *)