OFFSET
1,1
COMMENTS
Conjecture: all members of A045572 are in the sequence. - Robert Israel, Oct 24 2017
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
a(3)=7 => 97 is a prime but not necessarily 297 (in fact not a prime).
MAPLE
N:= 1000: # to get terms before the first term > N
P, C:= selectremove(isprime, [1, $3..N]):
dcat:= proc(x, y) 10^(1+ilog10(y))*x+y end proc:
A[1]:= 2:
for n from 2 do
if n::even then
for j from 1 to nops(C) do
if isprime(dcat(A[n-1], C[j])) then
A[n]:= C[j];
C:= subsop(j=NULL, C);
break
fi
od
else
for j from 1 to nops(P) do
if isprime(dcat(A[n-1], P[j])) then
A[n]:= P[j];
P:= subsop(j=NULL, P);
break
fi
od
fi;
if not assigned(A[n]) then break fi
od:
seq(A[i], i=1..n-1); # Robert Israel, Oct 24 2017
MATHEMATICA
p = Prime[ Range[ 500]]; np = Drop[ Complement[ Range[ 500], p], 1]; a[0] = 0; a[n_] := a[n] = Block[{k = 1, q = IntegerDigits[a[n - 1]]}, If[ OddQ[n], While[ !PrimeQ[ FromDigits[ Join[q, IntegerDigits[ p[[k]] ]]]], k++ ]; q = p[[k]]; p = Delete[p, k]; q, While[ !PrimeQ[ FromDigits[ Join[q, IntegerDigits[ np[[k]] ]]]], k++ ]; q = np[[k]]; np = Delete[np, k]; q]]; Table[ a[n], {n, 64}]
KEYWORD
nonn,base
AUTHOR
Robert G. Wilson v, Apr 23 2004
STATUS
approved