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A061870
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Numbers such that |first digit - second digit + third digit - fourth digit ...| = 1.
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28
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1, 10, 12, 21, 23, 32, 34, 43, 45, 54, 56, 65, 67, 76, 78, 87, 89, 98, 100, 111, 120, 122, 131, 133, 142, 144, 153, 155, 164, 166, 175, 177, 186, 188, 197, 199, 210, 221, 230, 232, 241, 243, 252, 254, 263, 265, 274, 276, 285, 287, 296, 298, 320, 331, 340, 342
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OFFSET
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1,2
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COMMENTS
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Multiples of 11 plus or minus 1. If 11k+1 is a perfect square (see A219257) then a(n) is the square root of 11k+1. [Gary Detlefs, Feb 22 2010]
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LINKS
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EXAMPLE
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120 is in the sequence since |1-2+0| = 1.
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MATHEMATICA
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fQ[n_] := Abs[Plus @@ (((-1)^Range[Floor[Log[10, n] + 1]])*IntegerDigits@n)] == 1; Select[ Range@342, fQ@# &]
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PROG
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(PARI) altsum(v)=sum(i=1, #v, v[i]*(-1)^i)
(Python)
def ok(n): return abs(sum(int(di)*(-1)**i for i, di in enumerate(str(n)))) == 1
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CROSSREFS
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KEYWORD
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nonn,base,easy
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AUTHOR
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STATUS
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approved
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