login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A006753 Smith (or joke) numbers: composite numbers n such that sum of digits of n = sum of digits of prime factors of n (counted with multiplicity).
(Formerly M3582)
67
4, 22, 27, 58, 85, 94, 121, 166, 202, 265, 274, 319, 346, 355, 378, 382, 391, 438, 454, 483, 517, 526, 535, 562, 576, 588, 627, 634, 636, 645, 648, 654, 663, 666, 690, 706, 728, 729, 762, 778, 825, 852, 861, 895, 913, 915, 922, 958, 985, 1086, 1111, 1165 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Of course primes also have this property, trivially.

a(133809) = 4937775 is the first Smith number historically: 4937775 = 3*5*5*65837 and 4+9+3+7+7+7+5 = 3+5+5+(6+5+8+3+7) = 42, Albert Wilansky coined the term Smith number when he noticed the defining property in the phone number of his brother-in-law Harold Smith: 493-7775.

There are 248483 7-digit Smith numbers, corresponding to US phone numbers without area codes (like 4937775). - Charles R Greathouse IV, May 19 2013

REFERENCES

M. Gardner, Penrose Tiles to Trapdoor Ciphers. Freeman, NY, 1989, p. 300.

R. K. Guy, Unsolved Problems in the Theory of Numbers, Section B49.

Oltikar, Sham and Keith Wayland. "Construction of Smith Numbers," Mathematics Magazine, vol. 56(1), 1983, pp. 36-37.

C. A. Pickover, "A Brief History of Smith Numbers" in "Wonders of Numbers: Adventures in Mathematics, Mind and Meaning", pp. 247-248, Oxford University Press, 2000.

J. E. Roberts, Lure of the Integers, pp. 269-270 MAA 1992.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

D. D. Spencer, Key Dates in Number Theory History, Camelot Pub. Co. FL, 1995, pp. 94.

David Wells, The Penguin Dictionary of Curious and Interesting Numbers (Rev. ed. 1997), p. 180.

A. Wilansky, Smith numbers, Two-Year Coll. Math. J., 13 (1982), 21.

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

K. S. Brown's Mathpages, Smith Numbers and Rhonda Numbers

C. K. Caldwell, The Prime Glossary, Smith number

P. J. Costello, Smith Numbers

S. S. Gupta, Smith Numbers

T. Jason, Smith number

Madras Math's Amazing Number Facts, Smith Numbers

C. A. Pickover, "Wonders of Numbers, Adventures in Mathematics, Mind and Meaning," Zentralblatt review

C. Rivera, PrimePuzzles.Net, Problem 107:Consecutive Smith numbers

C. Rivera, PrimePuzzles.Net, Problem 108:Methods for generating Smith numbers

W. Schneider, Smith Numbers

Eric Weisstein's World of Mathematics, Smith Number

Wikipedia, Smith number

FORMULA

A007953(a(n)) = sum(A007953(A027746(a(n),k)): k=1..A001222(a(n))) and A066247(a(n))=1. [Reinhard Zumkeller, Dec 19 2011]

EXAMPLE

58 = 2*29; sum of digits of 58 is 13, sum of digits of 2 + sum of digits of 29 = 2+11 is also 13.

MATHEMATICA

fQ[n_] := !PrimeQ@ n && n>1 && Plus @@ Flatten[ IntegerDigits@ Table[ #[[1]], {#[[2]] }] & /@ FactorInteger@ n] == Plus @@ IntegerDigits@ n; Select[ Range@ 1200, fQ]

PROG

(Sage) is_A006753 = lambda n: n > 1 and not is_prime(n) and sum(n.digits()) == sum(sum(p.digits())*m for p, m in factor(n)) # D. S. McNeil, Dec 28 2010

(Haskell)

a006753 n = a006753_list !! (n-1)

a006753_list = [x | x <- a002808_list,

                    a007953 x == sum (map a007953 (a027746_row x))]

-- Reinhard Zumkeller, Dec 19 2011

(PARI) dsum(n)=my(s); while(n, s+=n%10; n\=10); s

is(n)=if(isprime(n), 0, my(f=factor(n)); sum(i=1, #f[, 1], dsum(f[i, 1])*f[i, 2])==dsum(n))

\\ Charles R Greathouse IV, Jan 03 2012

CROSSREFS

Cf. A019506, A050224, A050255, A098834-A098840, A103123-A103126, A104166-A104171, A104390, A104391, A002808, A050218, A202387, A202388.

Sequence in context: A022385 A244411 A213240 * A098836 A204341 A036920

Adjacent sequences:  A006750 A006751 A006752 * A006754 A006755 A006756

KEYWORD

nonn,base,nice,easy

AUTHOR

N. J. A. Sloane.

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified August 24 19:53 EDT 2016. Contains 275781 sequences.