A278999 Numbers of the form (5h+2)*5^k-1 or (5h+3)*5^k-1 for h,k in N.

1, 2, 6, 7, 9, 11, 12, 14, 16, 17, 21, 22, 26, 27, 31, 32, 34, 36, 37, 39, 41, 42, 46, 47, 49, 51, 52, 56, 57, 59, 61, 62, 64, 66, 67, 71, 72, 74, 76, 77, 81, 82, 84, 86, 87, 89, 91, 92, 96, 97, 101, 102, 106, 107, 109, 111, 112, 114, 116, 117, 121, 122, 126, 127, 131

Offset

1,2

Links

Ray Chandler, Table of n, a(n) for n = 1..10000

Hao Fu and G.-N. Han, Computer assisted proof for Apwenian sequences related to Hankel determinants, arXiv preprint arXiv:1601.04370 [math.NT], 2016. See sequence "K" in Section 2.2.

Mathematica

isok[n_]:=Module[{ord=IntegerExponent[n+1, 5]}, MemberQ[{2, 3}, Mod[(n+1)/5^ord, 5]]]; Select[Range[0, 131], isok] (*Ray Chandler, Dec 17 2016*)

Prog

(Python)
from sympy import integer_log
def A278999(n):
    def f(x): return n-1+sum(((m:=(x+1)//5**i)-1)//5+(m-4)//5+2 for i in range(integer_log(x+1, 5)[0]+1))
    m, k = n, f(n)
    while m != k: m, k = k, f(k)
    return m # Chai Wah Wu, Feb 20 2025

Crossrefs

Complement of A278998.

Sequence in context: A047554 A206446 A079335 * A190784 A344167 A061416

Adjacent sequences: A278996 A278997 A278998 * A279000 A279001 A279002

Keyword

nonn

Author

N. J. A. Sloane, Dec 07 2016

Extensions

More terms from Ray Chandler, Dec 17 2016

Status

approved