A020946 a(n) is the smallest number k such that A002487(k) = n.

0, 1, 3, 5, 9, 11, 33, 19, 21, 35, 39, 37, 45, 43, 69, 73, 93, 77, 75, 83, 189, 85, 141, 139, 153, 151, 147, 155, 267, 149, 165, 173, 279, 275, 171, 283, 315, 277, 537, 325, 297, 293, 579, 301, 309, 365, 333, 299, 567, 331, 339, 553, 549, 563, 1275, 341, 585, 565, 615, 629

Offset: 0

Author

N. J. A. Sloane and David W. Wilson, Jun 27 2002

Keywords

• nonn

Examples

			A002487(33) = 6 and this is the first time 6 appears, so a(6) = 33.
		

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 0..10000

Crossrefs

Equals A020950 + 1.

Cf. A020943-A020945, A002487, A020947-A020950.

Programs

Mathematica

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 *)

PARI

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

Python

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