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A335667 a(n) is the smallest even number k such that k-1 and k+1 are both n-almost primes. 1
4, 34, 274, 2276, 8126, 184876, 446876, 18671876, 95234374, 1144976876, 6018359374, 281025390626, 2068291015624, 6254345703124 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

10^13 < a(15) <= 181171630859374. - Giovanni Resta, Jun 21 2020

LINKS

Table of n, a(n) for n=1..14.

David A. Corneth, Upper bounds on a(n) for n = 1..37 where values with * are proven values

EXAMPLE

a(1) = 4 since 4 - 1 and 4 + 1 are both primes.

a(2) = 34 since 34 - 1 = 33 = 3*11 and 34 + 1 = 35 = 5*7 are both semiprimes.

a(3) = 274 since 274 - 1 = 273 = 3*7*13 and 274 + 1 = 275 = 5^2 * 11 are both 3-almost primes.

MATHEMATICA

m = 8; v = Table[0, {m}]; c = 0; o1 = 1; n = 4; While[c < m, o2 = PrimeOmega[n + 1]; If[o1 == o2 && v[[o1]] == 0, c++; v[[o1]] = n]; o1 = o2; n += 2]; v

CROSSREFS

Cf. A001222, A088077, A115186, A154704.

Sequence in context: A198687 A116430 A216239 * A201684 A155628 A036352

Adjacent sequences:  A335664 A335665 A335666 * A335668 A335669 A335670

KEYWORD

nonn,more

AUTHOR

Zak Seidov and Amiram Eldar, Jun 17 2020

EXTENSIONS

a(12)-a(14) from Giovanni Resta, Jun 21 2020

STATUS

approved

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Last modified April 16 17:01 EDT 2021. Contains 343050 sequences. (Running on oeis4.)