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”).
%I #9 Sep 08 2022 08:45:15
%S 7849,3667649,91171007,2395420006033,11877172892329028459041,
%T 604107995057426434824791,107174878415004743976428761769,
%U 424678439961073471604787362241217,1983672219242345491970468171243171249,10788746499945827829225142589096882612369,42855626937384013751014398588294858582343260060671
%N Semiprimes of the form A007925(n) = n^(n+1)-(n+1)^n.
%e a(1)=7849 because 5^6-6^5=7849=47*167 is a semiprime.
%t Select[Table[n^(n + 1) - (n + 1)^n, {n, 30}], PrimeOmega[#] == 2&] (* _Vincenzo Librandi_, Sep 21 2012 *)
%o (Magma) IsSemiprime:=func<n | &+[d[2]: d in Factorization(n)] eq 2>; [s: n in [3..30] | IsSemiprime(s) where s is n^(n+1)-(n+1)^n]; // _Vincenzo Librandi_, Sep 21 2012
%Y Cf. A007925 n^(n+1)-(n+1)^n, A072179 n^(n+1)-(n+1)^n is prime, A099499 primes of the form n^(n+1)-(n+1)^n, A099497 n^(n+1)-(n+1)^n is a semiprime.
%K nonn
%O 1,1
%A _Hugo Pfoertner_, Oct 19 2004
%E a(9)-a(11) from _Vincenzo Librandi_, Sep 21 2012