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A188059 Numbers k with the property that k, k+1 and 2*k+1 are all semiprimes. 2
25, 34, 38, 57, 93, 118, 133, 145, 177, 201, 205, 213, 218, 298, 334, 361, 381, 394, 446, 501, 633, 694, 698, 842, 865, 878, 898, 921, 1114, 1141, 1226, 1285, 1293, 1465, 1513, 1654, 1713, 1726, 1761, 1857, 1893, 1941, 1981, 2018, 2041, 2217, 2306, 2426, 2433, 2577, 2581, 2734, 2746, 2901, 2973, 3133, 3193, 3214, 3241, 3386, 3578, 3661, 3693, 3746, 3754, 3777, 3826, 3957 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Numbers k such that 2k+1 is a semiprime and the sum of two consecutive semiprimes (k and k+1).
LINKS
Zak Seidov, Table of n, a(n) for n = 1..1309 (terms < 200000)
FORMULA
Equals A111153 intersect A070552. - M. F. Hasler, Mar 20 2011
EXAMPLE
25 is a term: k = 25 = 5*5, k+1 = 26 = 2*13, 2k+1 = 51 = 3*17.
MATHEMATICA
Select[Range[4000], Union[PrimeOmega[{#, #+1, 2 #+1}]]=={2}&] (* Harvey P. Dale, May 11 2012 *)
CROSSREFS
Cf. A001358 (semiprimes).
Cf. A176896 (safe semiprimes), A111153 (Sophie Germain semiprimes), A070552.
Sequence in context: A020258 A227516 A281369 * A257720 A098368 A281370
KEYWORD
nonn
AUTHOR
Zak Seidov, Mar 20 2011
STATUS
approved

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Last modified March 19 03:33 EDT 2024. Contains 370952 sequences. (Running on oeis4.)