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 A272078 Numbers k such that k^2 + 1 is product of 3 distinct primes. 2
 13, 17, 21, 23, 27, 31, 33, 37, 53, 55, 63, 67, 72, 75, 77, 81, 87, 89, 91, 97, 98, 103, 105, 109, 111, 112, 113, 115, 119, 122, 125, 127, 128, 129, 135, 137, 138, 142, 147, 148, 149, 151, 153, 155, 161, 162, 163, 167, 172, 174, 179, 185, 189, 192, 197, 200, 208 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Daniel Starodubtsev, Table of n, a(n) for n = 1..1000 EXAMPLE 13 appears in the list because 13^2 + 1 = 170 = 2 * 5 * 17. 21 appears in the list because 21^2 + 1 = 442 = 2 * 13 * 17. MATHEMATICA A272078 = {}; Do[ k = Last /@ FactorInteger[n^2 + 1]; If[k == {1, 1, 1}, AppendTo[A272078, n]], {n, 1000}]; A272078 Select[Range[1000], Last /@ FactorInteger[#^2 + 1] == {1, 1, 1} &] PROG (PARI) isok(k) = my(x=k^2+1); (omega(x)==3) && (bigomega(x)==3); \\ Michel Marcus, Mar 11 2020 CROSSREFS Cf. A007304, A046386, A176686, A176687. Sequence in context: A108560 A274422 A134257 * A205722 A105731 A060343 Adjacent sequences: A272075 A272076 A272077 * A272079 A272080 A272081 KEYWORD nonn AUTHOR K. D. Bajpai, Apr 19 2016 STATUS approved

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Last modified March 31 19:11 EDT 2023. Contains 361672 sequences. (Running on oeis4.)