[seqfan] Re: Insert "+" and always get a prime

[john.mason at lispa.it]

I suppose you could do the same with the minus sign, but meaning absolute
difference, rather than simple subtraction.
So 13 and many other 2-digit numbers would be included, and 114 as the
first (I think) 3-digit number.
John

Inviato da iPad

> Il giorno 22 Dec 2014, alle ore 15:47, "Eric Angelini"
<Eric.Angelini at kntv.be> ha scritto:
>
> Hello SeqFans,
> Look at those integers:
>
> 110,112,116,118,130,136
>
> You will always get a prime if you insert
> a "+" sign inside them, no matter where:
>
> 110 --> 1+10=11 and 11+0=11
> 112 --> 1+12=13 and 11+2=13
> 116 --> 17
> 118 --> 19
> 130 --> 1+30=31 and 13+0=13
> 136 --> 1+36=37 and 13+6=19
>
> 101 is not an integer with this property
> as 1+01=2 is not allowed (no substring
> starting with "0").
>
> I guess S, the sequence of integers that
> share this property) will start like this:
>
> S = 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, 110, 112,
116, 118, 130, 136,...
> Best,
> É.
>
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