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A055458
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a(n) = smallest composite solution x to the equation phi(x+2n) = phi(x)+2n.
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7
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6, 12, 21, 24, 36, 45, 48, 39, 63, 72, 72, 95, 60, 57, 224, 84, 15, 135, 1058, 45, 301, 144
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OFFSET
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1,1
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COMMENTS
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Sivaramakrishnan (1989) quotes Makowski, who gave solutions for phi(x+d) = phi(x)+d with d = 2^a and d = 2*3^a. Compare also A007694 and A049237.
Smallest prime solutions appear to be identical with A054906.
a(23) is presently unknown.
The sequence continues as (with ? for unknown values): ?, 95, 162, 63, 189, 69, 156, 161, 180, 69, 260, 150, ?, 115, 204, 129, 400, 75, 180, 165, 35, 117, 476, 7105, 288, 195, ?, 324, 620, 240, 81, 145, 14531, 153, 644, 309, ?, 203, ?, 63, 640, 75, 372, 285, 2312, 33, 343, 642, 336, 155, ?, 147, 728, 396, 1564, 185, 564, 87, 567, 360, 360, 155, 492, 510, 560, 516, 516, 301, 4232, 261, 860, 387, 576, 185, 564, 309, 1000, 225 ... - Don Reble, Apr 29 2015
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REFERENCES
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Sivaramakrishnan, R. (1989): Classical theory of Arithmetical Functions. Marcel Dekker, Inc., New York-Basel. Chapter V, Problem 20, page 113.
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LINKS
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EXAMPLE
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a(19) = 1058 because phi(1058 + 38) = phi(1096) = 544 = 506 + 38 = phi(1058) + 38.
a(100) = 225, phi(225 + 200) = phi(425) = 320 = 120 + 200 = phi(225) + 200.
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MAPLE
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local x;
for x from 0 do
if not isprime(x) then
if numtheory[phi](x+2*n) = numtheory[phi](x)+2*n then
return x;
end if;
end if;
end do:
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MATHEMATICA
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Table[k = 4; While[Nand[CompositeQ@ k, EulerPhi[k + 2 n] == EulerPhi[k] + 2 n], k++]; k, {n, 22}] (* Michael De Vlieger, Dec 17 2016 *)
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PROG
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(PARI) a(n)=forcomposite(x=4, , if(eulerphi(x+2*n) == eulerphi(x)+2*n, return(x))) \\ does not handle -1s; Charles R Greathouse IV, Apr 28 2015
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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