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A000221
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Take sum of squares of digits of previous term; start with 5.
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14
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5, 25, 29, 85, 89, 145, 42, 20, 4, 16, 37, 58, 89, 145, 42, 20, 4, 16, 37, 58, 89, 145, 42, 20, 4, 16, 37, 58, 89, 145, 42, 20, 4, 16, 37, 58, 89, 145, 42, 20, 4, 16, 37, 58, 89, 145, 42, 20, 4, 16, 37, 58, 89, 145, 42, 20, 4, 16, 37, 58, 89, 145, 42, 20, 4, 16, 37, 58, 89, 145
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,1
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COMMENTS
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As the orbit of 5 under A003132, this could as well start with index 0. - M. F. Hasler, Apr 27 2018
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REFERENCES
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R. Honsberger, Ingenuity in Mathematics, Random House, 1970, p. 83.
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LINKS
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FORMULA
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Ultimately periodic with period 8.
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MATHEMATICA
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NestList[Plus @@ IntegerDigits[ # ]^2 &, 5, 50]
PadRight[{5, 25, 29, 85}, 120, {4, 16, 37, 58, 89, 145, 42, 20}] (* Harvey P. Dale, Jan 14 2022 *)
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PROG
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(PARI) A000221(n)=[20, 4, 16, 37, 58, 89, 145, 42, 5, 25, 29, 85][n%8+8^(n<5)] \\ M. F. Hasler, May 24 2009, edited Apr 27 2018
(Magma) [5, 25, 29, 85] cat &cat[[89, 145, 42, 20, 4, 16, 37, 58]: n in [0..17]]; // Vincenzo Librandi, Jan 29 2013
(Haskell)
a000221 n = a000221_list !! (n-1)
a000221_list = iterate a003132 5
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CROSSREFS
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Cf. A003132 (the iterated map), A003621, A039943, A099645, A031176, A007770, A000216 (starting with 2), A000218 (starting with 3), A080709 (starting with 4), A008460 (starting with 6), A008462 (starting with 8), A008463 (starting with 9), A139566 (starting with 15), A122065 (starting with 74169). - M. F. Hasler, May 24 2009
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KEYWORD
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nonn,base,easy,nice
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AUTHOR
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STATUS
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approved
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