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A217837 Numbers n such that 4n - 1 and 4n + 1 are both semiprimes. 1
14, 23, 30, 36, 40, 46, 51, 54, 55, 59, 62, 72, 75, 76, 80, 82, 85, 98, 103, 104, 113, 118, 129, 132, 134, 136, 138, 145, 167, 170, 172, 174, 195, 198, 204, 225, 240, 264, 270, 275, 280, 284, 285, 287, 292, 301, 314, 318, 329, 336, 337, 339, 346, 347, 348 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This is to semiprimes as A045753 is to primes.

LINKS

Zak Seidov, Table of n, a(n) for n = 1..1000

EXAMPLE

a(1) = 14 because 56 - 1 = 5*11 and 56 + 1 = 3*19 are semiprimes, and this is the first number having this property.

a(2) = 23 because 92 - 1 = 7*13 and 92 + 1 = 3*31 are semiprimes, and this is the second number having this property.

a(3) = 30 because 120 - 1 = 7*17 and 120 + 1 = 11*11 are semiprimes, and this is the third number having this property.

MATHEMATICA

Select[Range[500], PrimeOmega[4# - 1] == 2 && PrimeOmega[4# + 1] == 2 &] (* Alonso del Arte, Oct 12 2012 *)

CROSSREFS

Cf. A045753.

Sequence in context: A133150 A005734 A003694 * A285248 A102876 A188166

Adjacent sequences:  A217834 A217835 A217836 * A217838 A217839 A217840

KEYWORD

nonn

AUTHOR

Zak Seidov, Oct 12 2012

STATUS

approved

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Last modified August 10 14:50 EDT 2020. Contains 336381 sequences. (Running on oeis4.)