A360505 Concatenate the ternary strings for n, n-1, n-2, ..., 2, 1.

1, 21, 1021, 111021, 12111021, 2012111021, 212012111021, 22212012111021, 10022212012111021, 10110022212012111021, 10210110022212012111021, 11010210110022212012111021, 11111010210110022212012111021, 11211111010210110022212012111021, 12011211111010210110022212012111021

Offset: 1

Author

N. J. A. Sloane, Feb 17 2023

Keywords

• nonn
• base

Comments

Similar to A360502, but here we count down from n to 1 in base 3 and concatenate the strings to get a(n).

This is the ternary analog of A000422.

Links

• Index entries for sequences related to Most Wanted Primes video

Crossrefs

Cf. A000422, A007809, A360502, A360506.

Programs

Mathematica

a[n_] := FromDigits @ Flatten @ IntegerDigits[Range[n, 1, -1], 3]; Array[a, 15] (* Amiram Eldar, Feb 18 2023 *)

PARI

a(n) = strjoin(concat([digits(k, 3) | k <- Vecrev([1..n])])) \\ Rémy Sigrist, Feb 18 2023

Python

from sympy.ntheory import digits
def a(n): return int("".join("".join(map(str, digits(k, 3)[1:])) for k in range(n, 0, -1)))
print([a(n) for n in range(1, 16)]) # Michael S. Branicky, Feb 18 2023

Python

# faster version for initial segment of sequence
from sympy.ntheory import digits
from itertools import count, islice
def agen(s=""): yield from (int(s:="".join(map(str, digits(n, 3)[1:]))+s) for n in count(1))
print(list(islice(agen(), 15))) # Michael S. Branicky, Feb 18 2023