A187234 - OEIS

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

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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: