login

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Admirable numbers in the middle of twin primes.
1

%I #8 Sep 11 2023 12:13:00

%S 12,30,42,102,138,270,282,618,642,822,1488,1698,1878,2082,2238,2382,

%T 2658,2802,3462,3558,3918,4638,4722,5442,6198,6702,8538,8598,9678,

%U 10938,12162,12378,12822,12918,13218,13722,13758,13998,14082,16062,17418

%N Admirable numbers in the middle of twin primes.

%C Numbers n such that n is admirable, n-1 is prime and n+1 is prime.

%H Amiram Eldar, <a href="/A135502/b135502.txt">Table of n, a(n) for n = 1..10000</a>

%F A014574 INTERSECT A111592. - _R. J. Mathar_, Feb 10 2008

%e 30 is in the sequence, as 29=30-1 and 31=30+1 are a pair of twin primes.

%t Select[Range[18000],MemberQ[Most[Divisors[#]],(DivisorSigma[1,#]-2#)/2]&&AllTrue[#+{1,-1},PrimeQ]&] (* _Harvey P. Dale_, Sep 11 2023 *)

%Y Cf. A111592, A001359, A006512.

%K easy,nonn

%O 1,1

%A _Olaf Voß_, Feb 09 2008