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A059868
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There exist no palindromic pentagonal numbers of length a(n).
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7
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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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A002069 = {0, 1, 5, 22, 1001, 2882, 15251, 720027, 7081807, 7451547, 26811862, 54177145, 1050660501, 1085885801, 1528888251, 2911771192, 2376574756732, 5792526252975, 5875432345785, 10810300301801, 264571020175462, 5292834004382925, 10808388588380801, 15017579397571051, 76318361016381367, 150621384483126051, 735960334433069537, 1003806742476083001, 1087959810189597801, 2716280733370826172};
A059868[n_] := Length[Select[A002069, 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 penpals(limit):
for k in range(limit+1):
if ispal(k*(3*k-1)//2): yield k*(3*k-1)//2
def aupto(limit):
lengths = set(range(1, limit+1))
for p in penpals(10**limit):
lp, minlen = len(str(p)), min(lengths)
for li in list(lengths):
if li < lp: print(li, "in A059868"); lengths.discard(li)
if lp in lengths: lengths.discard(lp); print("... discarding", lp)
if len(lengths) == 0: return
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CROSSREFS
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KEYWORD
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nonn,base,hard
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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