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A007770 Happy numbers: numbers whose trajectory under iteration of sum of squares of digits map (see A003132) includes 1. 60
1, 7, 10, 13, 19, 23, 28, 31, 32, 44, 49, 68, 70, 79, 82, 86, 91, 94, 97, 100, 103, 109, 129, 130, 133, 139, 167, 176, 188, 190, 192, 193, 203, 208, 219, 226, 230, 236, 239, 262, 263, 280, 291, 293, 301, 302, 310, 313, 319, 320, 326, 329, 331, 338 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Sometimes called friendly numbers, but this usage is deprecated.

Gilmer shows that the lower density of this sequence is < 0.1138 and the upper density is > 0.18577. - Charles R Greathouse IV, Dec 21 2011

Corrected the upper and lower density inequalities in the comment above. - Nathan Fox, Mar 14 2013

REFERENCES

L. E. Dickson, History of the Theory of Numbers, Vol, I: Divisibility and Primality, AMS Chelsea Publ., 1999.

R. K. Guy, Unsolved Problems Number Theory, Sect. E34.

J. N. Kapur, Reflections of a Mathematician, Chap. 34 pp. 319-324, Arya Book Depot New Delhi 1996.

LINKS

Jud McCranie, Table of n, a(n) for n = 1..143071

E. El-Sedy and S. Siksek, On happy numbers, Rocky Mountain J. Math. 30 (2000), 565-570.

Justin Gilmer, On the density of happy numbers (2011).

Hao Pan, Consecutive happy numbers

W. Schneider, Happy Numbers (Includes list of terms below 10000)

Eric Weisstein's World of Mathematics, Happy Number

Eric Weisstein's World of Mathematics, Digitaddition

Wikipedia, Happy number

FORMULA

From Ulrich Krug (leuchtfeuer37(AT)gmx.de), Apr 23 2009: (Start)

1) Every power 10^k is a member of the sequence.

2) If n is member the numbers obtained by placing zeros anywhere in n are members.

3) If n is member each permutation of digits of n gives another member.

4) If the repeated process of summing squared digits give a number which is already a member of sequence the starting number belongs to the sequence.

5) If n is a member the repunit consisting of n 1's is a member.

6) If n is a member delete any digit d, new number consisting of remaining digits of n and d^2 1's placed everywhere to n is a member.

7) It is conjectured that the sequence includes an infinite number of primes (see A035497).

8) For any starting number the repeated process of summing squared digits ends with 1 or gives an "8-loop" which ends with (37,58,89,145,42,20,4,16,37) (End)

Recurrence formula to test if a number is happy

f(1)=Number;

f(n)=Sum_{k=0,floor(log10(f(n-1)))}(floor(f(n-1)/10^k)-10*floor(f(n-1)/10^(k+1)))^(2). - José de Jesús Camacho Medina, Mar 29 2014

If f(n) produces '1' in their values, then the number is happy.

EXAMPLE

1 is OK. 2 --> 4 --> 16 --> 37 --> ... --> 4, which repeats with period 8, so never reaches 1, so 2 (and 4) are unhappy.

A correspondent suggested that 98 is happy, but it is not.

MATHEMATICA

f[n_] := Total[IntegerDigits[n]^2]; Select[Range[400], NestWhile[f, #, UnsameQ, All] == 1 &] (* T. D. Noe, Aug 22 2011 *)

Select[Range[1000], FixedPoint[Total[IntegerDigits[#]^2]&, #, 10]==1&] (* Harvey P. Dale, Oct 09 2011 *)

A example with recurrence formula to test if a number is happy:

a[1]=7;

a[n_]:=Sum[(Floor[a[n-1]/10^k]-10*Floor[a[n-1]/10^(k+1)]) ^ (2) , {k, 0,

      Floor[Log[10, a[n-1]]] }]

Table[a[n], {n, 1, 10}] (* José de Jesús Camacho Medina, Mar 29 2014 *)

PROG

(Haskell)

a007770 n = a007770_list !! (n-1)

a007770_list = filter ((== 1) . a103369) [1..]

-- Reinhard Zumkeller, Aug 24 2011

(PARI) ssd(n)=n=digits(n); sum(i=1, #n, n[i]^2)

is(n)=while(n>6, n=ssd(n)); n==1 \\ Charles R Greathouse IV, Nov 20 2012

CROSSREFS

Cf. A003132 (the underlying map), A001273, A035497 (happy primes), A046519, A031177, A002025, A050972, A050973, A074902, A003132, A103369, A035502, A068571, A072494, A124095, A219667.

Sequence in context: A026319 A120153 A226969 * A212979 A114961 A219045

Adjacent sequences:  A007767 A007768 A007769 * A007771 A007772 A007773

KEYWORD

nonn,base,nice,easy

AUTHOR

N. J. A. Sloane, A.R.McKenzie(AT)bnr.co.uk

STATUS

approved

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Last modified October 21 04:42 EDT 2014. Contains 248373 sequences.