A068892 Incorrect version of A085869.
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 13, 15, 17, 22, 26, 31, 33, 37, 39, 44, 51, 55, 62, 66, 71
Offset: 0
This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
15 is a term since 15 = 3*5 and 51 = 3*17, and both are of the form p*q where p and q are primes.
sig[n_] := Sort @ FactorInteger[n][[;;,2]]; Select[Range[300], !PalindromeQ[#] && sig[#] == sig[IntegerReverse[#]] &] (* Amiram Eldar, Dec 30 2020 *)
24 and 42 are members as 24 and 42 each have 8 divisors but with different prime signatures.
psQ[n_]:=Sort[Transpose[FactorInteger[n]][[2]]]!=Sort[Transpose[ FactorInteger[FromDigits[Reverse[IntegerDigits[n]]]]][[2]]];ndQ[n_]:= DivisorSigma[0,n]==DivisorSigma[0,FromDigits[Reverse[ IntegerDigits[ n]]]]; Select[Range[5000],psQ[#]&&ndQ[#]&] (* Harvey P. Dale, Nov 11 2011 *)
a(6) = 100006 = 2*13*1613 and 600001 = 19*23*1373 both have prime signature p*q*r, p,q,r are primes.
rev(n) = fromdigits(Vecrev(digits(n)));
psig(n) = vecsort(factor(n)[,2]~);
a(n) = {my(k = 10^n+1); while(k == rev(k) || psig(k) != psig(rev(k)), k++); k;} \\ Amiram Eldar, May 19 2024
Comments