A257842 Semiprimes p*q such that R(p*q) = R(p)*R(q), where R = A004086 = reverse digits.

4, 6, 9, 22, 26, 33, 39, 46, 55, 62, 69, 77, 82, 86, 93, 121, 143, 169, 187, 202, 206, 226, 253, 262, 299, 303, 309, 339, 341, 393, 422, 446, 451, 466, 473, 482, 505, 583, 622, 626, 633, 662, 669, 671, 699, 707, 781, 802, 842, 862, 866, 886, 933, 939, 961

Offset: 1

Author

M. F. Hasler, May 11 2015

Keywords

• nonn
• base

Comments

A subsequence of A161600. Almost all terms with less than 4 digits are either multiples of 2 or 3 or of 11.

Links

• Giovanni Resta, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A161600, A071786, A004086.

Programs

Maple

N:= 1000: # to get all terms <= N
digrev:= proc(n) local L,i;
L:= convert(n,base,10);
add(L[-i]*10^(i-1),i=1..nops(L))
end proc:
F:= proc(p,q) if digrev(p*q)=digrev(p)*digrev(q) then p*q else NULL fi end proc:
sort([seq(seq(F(Primes[i],q), q = select(`<=`,Primes[i..-1],N/Primes[i])), i=1..nops(Primes))]); # Robert Israel, May 14 2015

Mathematica

f[n_]:=FactorInteger[n][[1,1]];g[n_]:=FromDigits[Reverse[IntegerDigits[n]]];Select[Range@1000,PrimeOmega[#]==2&&g[f[#]*#/f[#]]==g[f[#]]*g[#/f[#]]&] (* Ivan N. Ianakiev, May 14 2015 *)

PARI

is(n)=bigomega(n)==2&&!eval(concat(Vecrev(Str(n"-"vecmin(n=factor(n)[,1])"*"vecmax(n)))))