A153446 Terms in A046034 which are pairwise products of terms in A046034.

25, 35, 75, 225, 275, 375, 525, 575, 2275, 2325, 2555, 2775, 3775, 5575, 5775, 7575, 7725, 7755, 22575, 22725, 23275, 23325, 23725, 25275, 25375, 25575, 25725, 27335, 27375, 27775, 32775, 37275, 37775, 52325, 53325, 55225, 55275, 55575, 57375

Offset

1,1

Comments

All terms are = 5 (mod 10).

Links

David W. Wilson, Table of n, a(n) for n = 1..10410

Example

25 = 5*5 = A046034(3)*A046034(3) = A046034(7);
35 = 5*7 = A046034(3)*A046034(4) = A046034(11);
75 = 3*25 = A046034(2)*A046034(7) = A046034(19);
225 = 3*75 = A046034(2)*A046034(19) = A046034(23);
275 = 5*55 = A046034(3)*A046034(15) = A046034(35).

Mathematica

Select[Flatten@ Table[FromDigits /@ Tuples[{2, 3, 5, 7}, n], {n, 5}], Function[k, Total@ Map[Times @@ # &, Boole@ Map[Total@ Pick[DigitCount@ #, {1, 0, 0, 1, 0, 1, 0, 1, 1, 1}, 1] == 0 &, Transpose@ {#, k/#} &@ Rest@ Take[#, Ceiling[Length[#]/2]] &@ Divisors@ k, {2}]] > 0]] (* Michael De Vlieger, Sep 19 2016 *)
id[n_]:=IntegerDigits[n]; pQ[n_]:=AllTrue[id[n], PrimeQ];
nQ[n_]:=Select[Times@@@Tuples[Select[Divisors[n], AllTrue[id[#], PrimeQ]&], 2], #==n&]
!={};
Select[Flatten@Table[FromDigits/@Tuples[{2, 3, 5, 7}, n], {n, 5}], pQ[#]&&nQ[#]&] (* Ivan N. Ianakiev, Jul 20 2022 *)

Crossrefs

Cf. A046034 (numbers with prime digits).

Sequence in context: A046423 A347612 A331808 * A039457 A243749 A331143

Adjacent sequences: A153443 A153444 A153445 * A153447 A153448 A153449

Keyword

base,nonn

Author

Zak Seidov, Dec 26 2008

Status

approved