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

1,1

Links

Table of n, a(n) for n=1..35.

Example

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.

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);

Crossrefs

Cf. A001358, A213025.

Sequence in context: A259944 A063652 A259886 * A032771 A269237 A200256

Adjacent sequences: A365219 A365220 A365221 * A365223 A365224 A365225

Keyword

nonn

Author

Zak Seidov and Robert Israel, Aug 26 2023

Status

approved