|
| |
|
|
A070272
|
|
Numbers n such that reverse(n) = phi(n) + sigma(n).
|
|
1
| | |
|
|
|
OFFSET
| 1,1
|
|
|
COMMENTS
| For n>0 5*(6*10^A056716(n)-1) is in this sequence. In fact if p = 6*10^n-1 is prime and n>0 (p>5) then m = 5*p is in the sequence. That's because phi(m) = phi(5*p) = 4*(6*10^n-2) = 24*10^n-8 and sigma(m)= 6*6*10^n, so phi(m) + sigma(m) = 6*10^(n+1)-8 = 5.(9)(n).2 = reversal(2.(9)(n).5) = reversal (3*10^(n+1)-5) = reversal(m)(dot between numbers means concatenation and "(9)(n)" means number of 9's is n). For example 299999995 is in the sequence because 6*10^7-1 is prime and 299999995 = 5*(6*10^7-1); 299999999995 is in sequence because 6*10^10-1 is prime and 299999999995 = 5*(6*10^10-1). Next term is greater than 80000000. - Farideh Firoozbakht (mymontain(AT)yahoo.com), Jan 11 2005
Next term is greater than 10^9. - Farideh Firoozbakht (mymontain(AT)yahoo.com), Jan 23 2005
|
|
|
EXAMPLE
| Reverse(275) = 572 = 200 + 372 = phi(275) + sigma(275).
|
|
|
MATHEMATICA
| Select[Range[10^6], FromDigits[Reverse[IntegerDigits[ # ]]] == EulerPhi[ # ] + DivisorSigma[1, # ] &]
|
|
|
CROSSREFS
| Cf. A056716, A102284.
Sequence in context: A171346 A001242 A023681 * A066127 A062377 A157489
Adjacent sequences: A070269 A070270 A070271 * A070273 A070274 A070275
|
|
|
KEYWORD
| base,nonn
|
|
|
AUTHOR
| Joseph L. Pe (joseph_l_pe(AT)hotmail.com), May 12 2002
|
|
|
EXTENSIONS
| One more term from Farideh Firoozbakht (mymontain(AT)yahoo.com), Jan 11 2005
More terms from Farideh Firoozbakht (mymontain(AT)yahoo.com), Jan 23 2005
|
| |
|
|
