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A220161
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a(n) = 1 + 2^(2^n) + 2^(2^(n+1)).
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5
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7, 21, 273, 65793, 4295032833, 18446744078004518913, 340282366920938463481821351505477763073, 115792089237316195423570985008687907853610267032561502502920958615344897851393
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OFFSET
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0,1
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COMMENTS
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For n >= 1, W. Sierpiński proves that a(n) is divisible by 21.
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REFERENCES
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Arthur Engel, Problem-Solving Strategies, Springer, 1998, pages 121-122 (E3, said to be a "recent competition problem from the former USSR").
W. Sierpiński, 250 Problems in Elementary Number Theory. New York: American Elsevier, 1970. Problem #123.
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LINKS
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FORMULA
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MATHEMATICA
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Table[1+2^(2^n)+2^(2^(n+1)), {n, 0, 7}] (* Harvey P. Dale, Dec 16 2015 *)
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PROG
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(PARI) vector(10, n, n--; 1 + 2^(2^n) + 2^(2^(n+1))) \\ G. C. Greubel, Aug 10 2018
(Magma) [1 + 2^(2^n) + 2^(2^(n+1)): n in [0..10]]; // G. C. Greubel, Aug 10 2018
(Python)
def a(n): return 1 + 2**(2**n) + 2**(2**(n+1))
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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