A108636 Semiprimes with even digits.

4, 6, 22, 26, 46, 62, 82, 86, 202, 206, 226, 262, 422, 446, 466, 482, 622, 626, 662, 802, 842, 862, 866, 886, 2026, 2042, 2062, 2066, 2206, 2246, 2402, 2426, 2446, 2462, 2602, 2606, 2642, 2846, 2866, 4006, 4022, 4222, 4226, 4262, 4282, 4286, 4406, 4426, 4442

Offset

1,1

Comments

Semiprimes with even digits are less numerous than those with odd digits, cf. A091296.

"Semiprimes with even digits are less numerous than those with odd digits" because (base 10): no integer after 10 can end in a 0 without being divisible by 2, 5 and at least one other prime; for a semiprime to end in 2, 4, 6, or 8 it must be divisible by 2 and a prime with almost as many digits as the semiprime (and primes get rarer as they get longer). - Jonathan Vos Post, Nov 07 2005 [Corrected by Pontus von Brömssen, Jan 20 2026]

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Formula

a(n) = 2*A036956(n). - Ctibor O. Zizka, Jan 20 2026

Maple

f:= proc(n) local L, x, i;
  L:= convert(n, base, 5);
  x:= 2*add(L[i]*10^(i-1), i=1..nops(L));
  if isprime(x/2) then x else NULL fi
end proc:
map(f, [$1..1000]); # Robert Israel, Oct 01 2024

Mathematica

Select[Range[6000], Plus@@Last/@FactorInteger[ # ]==2&&Union[EvenQ/@IntegerDigits[ # ]]=={True}&]

Crossrefs

Intersection of A001358 and A014263.

Cf. A091296, A036956.

Sequence in context: A286358 A088228 A272309 * A101143 A083157 A192154

Adjacent sequences: A108633 A108634 A108635 * A108637 A108638 A108639

Keyword

nonn,base,look

Author

Zak Seidov, Jun 14 2005

Status

approved