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A188466 Numbers n such that lambda(n) = lambda(n + lambda(n)). 1
1, 4, 6, 16, 36, 55, 78, 105, 124, 144, 171, 200, 253, 325, 406, 465, 666, 689, 715, 741, 915, 930, 990, 1027, 1081, 1136, 1240, 1421, 1448, 1610, 1653, 1711, 1752, 1764, 1800, 1827, 2211, 2352, 2448, 2667, 2800, 2835, 3403, 3600, 3619, 3620, 3660, 3900, 4840, 4970, 5253, 5264, 5513, 5671, 5886, 6100, 6328, 8001, 8112 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Lambda is the function (A002322). If there are infinitely many Sophie Germain primes (conjecture), then this sequence is infinite. Proof: The numbers of the  form p(2p+1) are in a subsequence if p and 2p+1 are both prime with p > 3, because from the property that lambda(p(2p+1)) = p(p-1), if m = p(2p+1) then lambda(m+phi(m))  = lambda (p(2p+1) + p(p-1)) = lambda(3p^2) = p(p-1) = lambda(m).

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

EXAMPLE

36 is in the sequence because lambda(36) = 6, and lambda(36 + 6) = lambda(42) = 6.

MATHEMATICA

Select[Range[20000], CarmichaelLambda[ #] == CarmichaelLambda[ # + CarmichaelLambda[#]  ] &]

PROG

(MAGMA) [1] cat [n: n in [2..8140] | CarmichaelLambda(n) eq CarmichaelLambda(n+CarmichaelLambda(n))];  // Bruno Berselli, Apr 10 2011

(PARI) lambda(n) = lcm(znstar(n)[2]);

isok(n) = lambda(n) == lambda(n+lambda(n)); \\ Michel Marcus, May 12 2018

CROSSREFS

Cf. A002322, A005384.

Cf. A185165: Numbers n such that lambda(n)= lambda(n - lambda(n)).

Cf. A051487: Numbers n such that phi(n) = phi(n - phi(n)).

Cf. A108569: Numbers n such that phi(n) = phi(n + phi(n)).

Sequence in context: A113883 A036748 A162485 * A076066 A227178 A165799

Adjacent sequences:  A188463 A188464 A188465 * A188467 A188468 A188469

KEYWORD

nonn

AUTHOR

Michel Lagneau, Apr 01 2011

STATUS

approved

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Last modified November 27 03:27 EST 2020. Contains 338677 sequences. (Running on oeis4.)