A389007 Semiprimes whose least decimal digit is 2.

22, 25, 26, 62, 82, 226, 235, 237, 247, 249, 253, 254, 259, 262, 265, 267, 274, 278, 287, 289, 295, 298, 299, 323, 326, 327, 329, 362, 382, 422, 427, 482, 526, 527, 529, 542, 562, 622, 623, 626, 629, 662, 723, 842, 862, 922, 923, 926, 982, 2227, 2229, 2234, 2245, 2246, 2249, 2253, 2257, 2258

Offset

1,1

Links

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

Example

a(5) = 82 is a term because its least decimal digit is 2 and 82 = 2*41 is a semiprime.

Maple

S:= {2}: T:= {$3..9}: R:= NULL:
for d from 2 to 4 do
  S:= map(t -> seq(10*t+i, i=2..9), S) union map(t -> 10*t+2, T);
  T:= map(t -> seq(10*t+i, i=3..9), T);
  R:= R, op(sort(convert(select(s -> numtheory:-bigomega(s)=2, S), list)));
od:
R;

Mathematica

A389007Q[k_] := Min[IntegerDigits[k]] == 2 && PrimeOmega[k] == 2;
Select[Range[2500], A389007Q] (* Paolo Xausa, Sep 26 2025 *)

Crossrefs

Subset of A084374. Intersection of A001358 and A284063. Cf. A106101.

Sequence in context: A177734 A177055 A186780 * A034304 A167459 A066737

Adjacent sequences: A389004 A389005 A389006 * A389008 A389009 A389010

Keyword

nonn,base

Author

Robert Israel, Sep 22 2025

Status

approved