login
Semiprimes which remain semiprimes when the leftmost digit is removed.
3

%I #15 Jun 07 2017 09:33:29

%S 14,26,34,39,46,49,69,74,86,94,106,115,121,122,133,134,146,155,158,

%T 169,177,185,187,194,206,209,214,215,221,226,235,249,262,265,274,287,

%U 291,295,309,314,321,326,334,335,339,346,355,358,362,365,377,382,386,391

%N Semiprimes which remain semiprimes when the leftmost digit is removed.

%H Harvey P. Dale, <a href="/A235688/b235688.txt">Table of n, a(n) for n = 1..1000</a>

%e 249 is in the sequence because 49 = 7*7.

%t Select[Range[400],PrimeOmega[#]==PrimeOmega[Mod[#,10^(IntegerLength[ #]-1)]] == 2&] (* _Harvey P. Dale_, Jun 07 2017 *)

%o (PARI)

%o list(lim)=my(v=List(), t); forprime(p=2, sqrt(lim), t=p; forprime(q=p, lim\t, listput(v, t*q))); vecsort(Vec(v)) \\ From A001358

%o delleft(a) = my(b, c); b=#Str(a); c=a\(10^(b-1)); a-c*(10^(b-1))

%o issemiprime(n) = n>0 && bigomega(n)==2

%o t=list(500); for(n=1, #t, if(issemiprime(delleft(t[n])), print1(t[n],", ")))

%Y Cf. A235687, A235689.

%Y Cf. A227919, A227916, A069686.

%K nonn,base

%O 1,1

%A _Colin Barker_, Jan 14 2014