A187234 Numbers such that Sum_{i<j, i,j =1..r} d(i)*d(j) is a prime number, where the d(i) are digits of n (n = concatenation of d(1), ..., d(r)).

12, 13, 15, 17, 21, 31, 51, 71, 102, 103, 105, 107, 111, 112, 113, 115, 116, 118, 119, 120, 121, 123, 125, 127, 129, 130, 131, 132, 134, 135, 137, 143, 145, 150, 151, 152, 153, 154, 156, 157, 158, 159, 161, 165, 170, 172, 173, 175, 178, 179, 181, 185, 187, 189, 191, 192, 195, 197, 198, 201, 210, 211, 213, 215, 217, 219, 231, 235, 237, 251, 253, 257

Offset

1,1

Links

Table of n, a(n) for n=1..72.

Example

253 is in the sequence because d(1)*d(2) + d(1)*d(3) + d(2)*d(3) = 2*5+2*3+5*3 = 31 is prime.

Maple

with(numtheory):T:=array(1..10):k:=1:for n from 1 to 1000 do:l:=length(n):n0:=n:for
  m from 1 to l do:q:=n0:u:=irem(q, 10):v:=iquo(q, 10):n0:=v :T[m]:=u:od: s:=0:
  for i from 1 to l-1 do: for j from i+1 to l do: s:=s+T[i]*T[j]:od: od:if type(s, prime)
  = true then printf(`%d, `, n):else fi:od:

Crossrefs

Cf. A187559.

Sequence in context: A085520 A085556 A175225 * A127354 A226099 A179512

Adjacent sequences: A187231 A187232 A187233 * A187235 A187236 A187237

Keyword

nonn,base,easy

Author

Michel Lagneau, Mar 11 2011

Status

approved