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A301868
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Numbers k such that phi(k, 2) = phi(k+1, 2), where phi(k, 2) = A002472(k).
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1
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1, 2, 9, 34, 50, 153, 274, 321, 2841, 4610, 7474, 8254, 10250, 13430, 22149, 38961, 51981, 86845, 91310, 198057, 237325, 367629, 374541, 394834, 419169, 489445, 513890, 516350, 519230, 570230, 717969, 1308609, 1523630, 1557909, 1753730, 1935362, 2109969, 3005409
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OFFSET
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1,2
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LINKS
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EXAMPLE
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phi(9, 2) = phi(10, 2) = 3, thus 9 is in the sequence.
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MATHEMATICA
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seq = {}; a[n_] := If[Head[r = Reduce[GCD[x, n] == 1 && GCD[x + 2, n] == 1 && 1 <= x <= n, x, Integers]] === Or, Length[r], 1]; a0 = a[1]; Do[
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PROG
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(PARI) f(n) = sum(x=1, n, (gcd(n, x) == 1) && (gcd(n, x+2) == 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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EXTENSIONS
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STATUS
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approved
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