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A177516 Odd numbers m = p*q such that p and q are distinct primes and (p-1) divides (q-1). 4
15, 21, 33, 39, 51, 57, 65, 69, 85, 87, 91, 93, 111, 123, 129, 133, 141, 145, 159, 177, 183, 185, 201, 205, 213, 217, 219, 237, 249, 259, 265, 267, 291, 301, 303, 305, 309, 321, 327, 339, 341, 365, 381, 393, 411, 417, 427, 445, 447, 451, 453, 469, 471, 481 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Neither p nor q can equal 2, i.e., 2 is not a permissible prime here. - Harvey P. Dale, May 04 2011

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

EXAMPLE

(3 - 1) divides (5 - 1), so 3 * 5 = 15 is in this sequence.

(5 - 1) divides (17 - 1) so 5 * 17 = 85 is in this sequence.

(5 - 1) does not divide (19 - 1), so 5 * 19 = 95 is not in this sequence.

MATHEMATICA

Table[If[i < j && IntegerQ[(Prime[j] - 1)/(Prime[i] - 1)], Prime[j] * Prime[i]], {i, 2, 100}, {j, 2, 100}] // Flatten // Union

Union[Times@@@Select[Subsets[Prime[Range[2, 100]], {2}], Divisible[Last[#] - 1, First[#] - 1] &]] (* Harvey P. Dale, May 04 2011 *)

CROSSREFS

Cf. A046388.

Sequence in context: A198680 A300117 A214044 * A127329 A043326 A179996

Adjacent sequences:  A177513 A177514 A177515 * A177517 A177518 A177519

KEYWORD

nonn

AUTHOR

José María Grau Ribas, Dec 11 2010

STATUS

approved

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Last modified October 2 06:08 EDT 2022. Contains 357191 sequences. (Running on oeis4.)