OFFSET
1,2
COMMENTS
The nonprime digits are 0, 1, 4, 6, 8 and 9. This is the lexicographically earliest sequence of distinct positive terms with this property.
LINKS
Robert Israel, Table of n, a(n) for n = 1..6500
EXAMPLE
a(1) * a(2) = 1 * 2 = 2 (no nonprime digit is present);
a(2) * a(3) = 2 * 11 = 22 (no nonprime digit is present);
a(3) * a(4) = 11 * 3 = 33 (no nonprime digit is present);
a(4) * a(5) = 3 * 9 = 27 (no nonprime digit is present); etc.
MAPLE
N:= 500: # for terms before the first term >= N
S:= select(t -> t mod 10 <> 0, [$2...N]):
nS:= nops(S):
V:=Vector(N):
V[1]:= 1:
for n from 2 do
for i from 1 to nS+2-n do
s:= S[i];
if convert(convert(V[n-1]*s, base, 10), set) subset {2, 3, 5, 7} then
V[n]:= s;
S:= subsop(i=NULL, S);
break
fi;
od;
if V[n] = 0 then break fi;
od:
convert(V[1..n-1], list); # Robert Israel, Nov 18 2020
MATHEMATICA
Block[{a = {1}}, Do[Block[{k = 1}, While[Nand[FreeQ[a, k], NoneTrue[IntegerDigits@ Total[a[[-1]]*k], ! PrimeQ@ # &]], k++]; AppendTo[a, k]], {i, 2, 76}]; a] (* Michael De Vlieger, Nov 12 2020 *)
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Eric Angelini and Carole Dubois, Nov 11 2020
STATUS
approved