A135355 - OEIS

%I #12 Jan 16 2023 11:10:07

%S 4,6,9,14,15,25,26,34,35,38,39,46,49,57,58,69,123,129,134,145,146,158,

%T 159,169,178,235,237,247,249,259,267,278,289,346,358,458,469,478,489,

%U 579,589,679,689,789,1234,1238,1247,1257,1267,1345,1346,1347,1349,1357

%N Semiprimes with digits in ascending order.

%C This is to A028864 as A001358 is to A000040. Semiprime metadromes in base 10. Contains 175 terms, the last being a(175)=12345789 = 3 * 4115263.

%C Snapshots: a(100)=13579, a(150)=135689, a(160)=245678, a(163)=356789, a(169)=1346789, a(173)=2356789. - _R. J. Mathar_, Dec 11 2007

%H R. J. Mathar, <a href="/A135355/b135355.txt">Table of n, a(n) for n = 1..175</a>

%F Equals A001358 INTERSECT A009993. - _R. J. Mathar_, Dec 11 2007

%p isA001358 := proc(n) if numtheory[bigomega](n) = 2 then true; else false ; fi ; end: isA009993 := proc(n) local dgs,i ; dgs := convert(n,base,10) ; if nops(dgs) = 1 then RETURN(true) ; fi ; for i from 1 to nops(dgs)-1 do if op(i,dgs) <= op(i+1,dgs) then RETURN(false) ; fi ; od: RETURN(true) ; end: isA135355 := proc(n) isA001358(n) and isA009993(n) ; end: for n from 4 to 1400 do if isA135355(n) then printf("%d, ",n) ; fi ; od: # _R. J. Mathar_, Dec 11 2007

%Y Cf. A001358, A028864.

%K base,easy,fini,nonn

%O 1,1

%A _Jonathan Vos Post_, Dec 08 2007

%E Corrected by _R. J. Mathar_, Dec 11 2007