OFFSET
1,1
COMMENTS
It appears that all a(n), n > 12, have initial digits "799".
The first digit of a(n) is never 9. - Chai Wah Wu, Sep 08 2022
As in A347475, all terms with more than 2 digits end in 33, 37, 73 or 77. - M. F. Hasler, Sep 12 2022
LINKS
M. F. Hasler, Table of n, a(n) for n = 1..24, Sep 08 2022
S. S. Gupta, Can You Find (CYF) no. 55, Nov 11 2021.
FORMULA
a(n) = max { k in A347475 | k < 10^n }.
EXAMPLE
T(5) = A000217(5) = 5*6/2 = 5*3 = 15 has only odd digits, and neither T(7) nor T(9) have this property, therefore a(1) = 5.
PROG
(Python)
from itertools import product
def A355277(n):
for a in '7531':
for b in product('97531', repeat=n-1):
m = int(a+''.join(b))
if set(str(m*(m+1)>>1)) <= {'1', '3', '5', '7', '9'}:
return m # Chai Wah Wu, Sep 08 2022
CROSSREFS
KEYWORD
nonn,base
AUTHOR
M. F. Hasler, Sep 07 2022
STATUS
approved