OFFSET
1,1
LINKS
P. De Geest, Palindromic hexagonals
MATHEMATICA
A054969 = {0, 1, 6, 66, 3003, 5995, 15051, 66066, 617716, 828828, 1269621, 1680861, 5073705, 5676765, 1264114621, 5289009825, 6172882716, 13953435931, 1313207023131, 5250178710525, 6874200024786, 61728399382716, 602224464422206, 636188414881636, 1250444114440521, 16588189498188561, 58183932923938185, 66056806460865066, 67898244444289876, 514816979979618415, 3075488771778845703, 6364000440440004636, 15199896744769899151};
Select[Range[19], A082721[#] == 0 &] (* Robert Price, Apr 27 2019 *)
PROG
(Python)
def ispal(n): s = str(n); return s == s[::-1]
def hexpals(limit):
yield from (k*(2*k-1) for k in range(limit+1) if ispal(k*(2*k-1)))
def aupto(limit):
lengths = set(range(1, limit+1))
for h in hexpals(10**limit):
if len(lengths) == 0: return
lh, minlen = len(str(h)), min(lengths)
if lh > minlen: print(minlen, "in A082721"); lengths.discard(minlen)
if lh in lengths: lengths.discard(lh); print("... discarding", lh)
aupto(14) # Michael S. Branicky, Mar 08 2021
CROSSREFS
KEYWORD
nonn,base,hard,more
AUTHOR
Patrick De Geest, Apr 13 2003
STATUS
approved