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A082721
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There exist no palindromic hexagonals of length n.
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6
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3, 8, 9, 12, 22, 24, 27, 30, 36, 38, 40
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,1
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LINKS
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MATHEMATICA
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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};
A082721[n_] := Length[Select[A054969, IntegerLength[#] == n || (n == 1 && # == 0) &]];
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PROG
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(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)
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CROSSREFS
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KEYWORD
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nonn,base,hard,more
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AUTHOR
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STATUS
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approved
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