[seqfan] Insert a "plus" sign to produce a prime

Eric Angelini Eric.Angelini at kntv.be
Fri Nov 14 11:32:15 CET 2014


Hello Seqfans,
Insert a "plus" sign somewhere in a(n): the addition produces a prime.

A = 11, 12, 14, 16, 20, 21, 23, 25, 29, 30, 32, 34, 38, 41, 43, 47, 49, 
    50, 52, 56, 58, 61, 65, 67, 70, 74, 76, 83, 85, 89, 92, 94, 98, 101, 
    103, 107, 109, 110, 112, 116, 118, 121, ...

Examples:

 11 is in A because 1+1 produces 2, which is prime
 12 is in A because 1+2 produces 3, which is prime
 ...
 98 is in A because 9+8 produces 17, which is prime
 99 is not in A because 9+9=18 is not prime
...
101 is in A because 10+1 produces 11, which is prime
102 is not in A because 10+2=12 is not prime AND
    because 1+02 has no meaning (no leading zeros
    admitted, nowhere)
Best,
É.
-----
P.-S.
The D sequence of primes produced by a "minus" sign starts like this:
D = 20, 30, 31, 41, 42, 50, 52, 53, 61, 63, 64, 70, ...




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