OFFSET
1,1
EXAMPLE
a(3) = 51 because 51 = 3 * 17 is a semiprime and 50 - 1 = 50 = 2 * 5^2 and 51 + 1 = 52 = 2^2 * 13 are triprimes.
MAPLE
V:= Vector(10): count:= 0:
b:= 1: c:= 2:
for x from 5 while count < 10 do
a:= b; b:= c; c:= numtheory:-bigomega(x);
if b = 2 and a = c and V[a] = 0 then
count:= count+1; V[a]:= x-1; printf("%d %d\n", a, x-1);
fi
od:
convert(V, list);
MATHEMATICA
seq[len_, kmax_] := Module[{s = Table[0, {len}], k = 2, c = 0, m}, While[c < len && k < kmax, If[PrimeOmega[k] == 2, m = PrimeOmega[k - 1]; If[m <= len && s[[m]] == 0 && PrimeOmega[k + 1] == m, c++; s[[m]] = k]]; k++]; s]; seq[10, 10^7] (* Amiram Eldar, Sep 08 2023 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov and Robert Israel, Sep 08 2023
EXTENSIONS
a(14) from Amiram Eldar, Sep 08 2023
a(15)-a(23) from Martin Ehrenstein, Dec 26 2023
STATUS
approved