# [seqfan] Re: Insert a "plus" sign to produce a prime

Felix Fröhlich felix.froe at googlemail.com
Fri Nov 14 13:19:22 CET 2014

```Hi Eric,

This is a nice sequence I think. Is the definition such that at least one
such prime must exist or do all numbers produced by all possible plus sign
insertions have to be prime. I assume the former, since in the case of 121,
1+21=22 is not prime. Maybe the definition could be made a bit more precise.

Best,

Felix

2014-11-14 11:32 GMT+01:00 Eric Angelini <Eric.Angelini at kntv.be>:

> 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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>
> Seqfan Mailing list - http://list.seqfan.eu/
>
```

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