A089754 Smallest prime ending (least significant side) in n if possible else beginning in n.

11, 2, 3, 41, 5, 61, 7, 83, 19, 101, 11, 127, 13, 149, 151, 163, 17, 181, 19, 2003, 421, 223, 23, 241, 251, 263, 127, 281, 29, 307, 31, 3203, 233, 347, 353, 367, 37, 383, 139, 401, 41, 421, 43, 443, 457, 461, 47, 487, 149, 503, 151, 521, 53, 541, 557, 563, 157, 587, 59, 601, 61, 6203, 163

Offset: 1

Author

Amarnath Murthy, Nov 22 2003

Keywords

• base
• easy
• nonn

Crossrefs

Cf. A018800, A062584, A068164.

Programs

Maple

read("transforms)"):
nleadzero := proc(n,dgs)
    local ndgs ;
    if n = 0 then
        ndgs := 0 ;
    else
        ndgs := ilog10(n)+1 ;
    end if;
    [op(convert(n,base,10)),seq(0,i=1..dgs-ndgs)]
end proc:
digcatL2 := proc(L1,L2)
    digcatL([op(L1),op(L2)]) ;
end proc:
A089754 := proc(n)
    local prep,a,dgs ;
    if isprime(n) then
        n;
    elif modp(n,2) = 0 or modp(n,5) = 0 then
        ndgs := convert(n,base,10) ;
        for dgs from 1 do
            for prep from 0 to 10^dgs-1 do
                a := digcatL2(ListTools[Reverse](ndgs),ListTools[Reverse](nleadzero(prep,dgs))) ;
                if isprime(a) then
                    return a;
                end if;
            end do:
        end do:
    else
        for prep from 1 do
            a := digcat2(prep,n) ;
            if isprime(a) then
                return a;
            end if;
        end do:
    end if;
end proc: # R. J. Mathar, Sep 18 2015

Extensions

Corrected by R. J. Mathar, Sep 18 2015