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A089653 (Product of prime factors of n) - 1 is a perfect square. 3
2, 4, 5, 8, 10, 16, 17, 20, 25, 26, 32, 37, 40, 50, 52, 64, 65, 80, 82, 100, 101, 104, 122, 125, 128, 145, 160, 164, 170, 197, 200, 208, 226, 244, 250, 256, 257, 289, 290, 320, 325, 328, 338, 340, 362, 400, 401, 416, 442, 452, 485, 488, 500, 512, 530, 577, 580 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..57.

EXAMPLE

The prime factors of 10 are 2 and 5 and 2 * 5 - 1 = 9 is a square; so 10 belongs to the sequence.

MATHEMATICA

ppf[n_] := Apply[Times, Transpose[FactorInteger[n]][[1]]]; Select[Range[2, 10^3], IntegerQ[Sqrt[ppf[ # ] - 1]] &]

CROSSREFS

Cf. A089632. A091419 gives squares, A091420 gives square roots.

Sequence in context: A199799 A032937 A126684 * A180252 A191203 A216686

Adjacent sequences:  A089650 A089651 A089652 * A089654 A089655 A089656

KEYWORD

nonn

AUTHOR

Joseph L. Pe, Jan 04 2004

STATUS

approved

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Last modified October 21 23:02 EDT 2018. Contains 316431 sequences. (Running on oeis4.)