OFFSET
1,1
COMMENTS
Concatenation is done with smaller factors to the left of larger factors.
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
EXAMPLE
105 is in the sequence because the prime distinct divisors of 105 are {3,5,7} and 357 = 3*7*17 is composite.
MAPLE
with(numtheory):for n from 1 to 200 do:x:=factorset(n):n1:=nops(x): s:=0:s0:=0:for i from n1 by -1 to 1 do: a:=x[i]:b:=length(a):s:=s+a*10^s0:s0:=s0+b:od: if type(s, prime)=false then printf(`%d, `, n):else fi:od:
PROG
(Magma) [n: n in [2..144] | not IsPrime(t) where t is Seqint(Reverse(&cat[Reverse(Intseq(PrimeDivisors(n)[k])): k in [1..#PrimeDivisors(n)]]))]; // Bruno Berselli, Mar 20 2012
(PARI) cat(n)=my(f=factor(n), s=""); for(i=1, #f[, 1], s=Str(s, f[i, 1])); eval(s)
p=7; forprime(q=11, 1e3, for(n=p+1, q-1, if(!isprime(cat(n)), print1(n", "))); p=q) \\ Charles R Greathouse IV, Mar 20 2012
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Michel Lagneau, Mar 13 2012
STATUS
approved