login
This site is supported by donations to The OEIS Foundation.

 

Logo

Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing.
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A276983 Semiprimes n such that n-1 or n+1 is prime. 1
4, 6, 10, 14, 22, 38, 46, 58, 62, 74, 82, 106, 158, 166, 178, 194, 226, 262, 278, 314, 346, 358, 382, 398, 422, 458, 466, 478, 502, 542, 562, 586, 614, 662, 674, 718, 734, 758, 838, 862, 878, 886, 982, 998, 1018, 1094, 1154, 1186, 1202, 1214, 1238, 1282, 1306, 1318, 1322 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Union of A077065 and A077068.

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = 2*A120628(n).

EXAMPLE

a(3) = 10 = 2*5 is a product of 2 primes and 10+1 = 11 is prime.

a(4) = 14 = 2*7 is a product of 2 primes and 14-1 = 13 is prime.

MAPLE

select(t -> isprime(t/2) and (isprime(t-1) or isprime(t+1)), [seq(i, i=2..10000, 2)]); # Robert Israel, Sep 30 2016

MATHEMATICA

func[n_] := PrimeOmega[n] == 2 && (PrimeQ[n + 1] || PrimeQ[n - 1])

Select[Range[1000], func[#] &]

PROG

(PARI) isok(n) = (bigomega(n)==2) && (isprime(n-1) || isprime(n+1)); \\ Michel Marcus, Sep 24 2016

(PARI) lista(nn) = forprime(p=2, nn, if(isprime(2*p+1) || isprime(2*p-1), print1(2*p, ", "))); \\ Altug Alkan, Sep 30 2016

CROSSREFS

Cf. A001358, A077065, A077068, A120628.

Sequence in context: A141247 A049632 A216732 * A061227 A274522 A000066

Adjacent sequences:  A276980 A276981 A276982 * A276984 A276985 A276986

KEYWORD

nonn

AUTHOR

Gary E. Davis, Sep 24 2016

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified December 6 18:03 EST 2019. Contains 329809 sequences. (Running on oeis4.)