A368337 Semiprimes that contain only digits 4 and 9.

4, 9, 49, 94, 949, 4449, 4499, 9449, 44494, 44949, 44999, 49949, 94499, 94994, 99449, 99494, 99949, 444494, 444949, 494449, 494999, 499949, 944494, 944949, 944999, 949999, 994999, 999494, 4444449, 4444499, 4449949, 4449999, 4494449, 4494499, 4494949, 4494999, 4499449, 4499494, 4944449, 4944499

Offset

1,1

Comments

The only terms that are squares are 4, 9 and 49.

Numbers of n-digit terms for n = 1...20: {2, 2, 1, 3, 13, 11, 31, 39, 78, 159, 383, 541, 1302, 2047, 4268, 6926, 16248, 27172, 57397, 94581}.

Links

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

Example

a(3) = 49 is a term because 49 = 7^2 is a semiprime with digits 4 and 9.

Maple

R:= 4, 9:
for d from 2 to 6 do
  for x from 0 to 2^d-1 do
    L:= convert(2^d+x, base, 2)[1..d];
    y:= add((L[i]*5+4)*10^(i-1), i=1..d);
    if numtheory:-bigomega(y)=2 then R:= R, y; fi
od od:
R;

Crossrefs

Intersection of A001358 and A284973.

Cf. A020466.

Sequence in context: A029791 A053961 A055812 * A268099 A061867 A019544

Adjacent sequences: A368334 A368335 A368336 * A368338 A368339 A368340

Keyword

nonn,base

Author

Zak Seidov and Robert Israel, Dec 21 2023

Status

approved