A365222 a(n) is the least semiprime such that a(n) - n and a(n) + n are the previous and the next semiprimes.

34, 185, 262, 407, 314, 371, 194, 2271, 6218, 4237, 109898, 110645, 53602, 169773, 112298, 163985, 284738, 48529, 1033378, 1781833, 570502, 1963091, 12527458, 6051613, 30377422, 19549343, 66761746, 7926901, 363311378, 5861227, 676386278, 136503631, 72622874, 51204973, 375025874

Offset: 1

Author

Zak Seidov and Robert Israel, Aug 26 2023

Keywords

• nonn

Examples

			a(3) = 262 because 262 = 2 * 131 is a semiprime, the previous semiprime is 262 - 3 = 259 = 7 * 37, the next semiprime is 262 + 3 = 265 = 5 * 53, and no semiprime less than 262 works.
		

Crossrefs

Cf. A001358, A213025.

Programs

Maple

N:= 46: # for a(1) .. a(N)
V:= Vector(N):
b:= 6: d:= 2: count:= 0:
for i from 7 while count < N do
  if numtheory:-bigomega(i) = 2 then
    ib:= i-b;
    if ib = d and ib <= N then
      if V[d] = 0 then V[d]:= b; count:= count+1 fi
    else d:= ib
    fi;
    b:= i;
  fi;
od:
convert(V,list);