

A007770


Happy numbers: numbers whose trajectory under iteration of sum of squares of digits map (see A003132) includes 1.


83



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
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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
Grundman defines the heights of the happy numbers by the number of iterations needed to reach the 1: 0, 5, 1, 2, 4, 3, 3, 2, 3, 4, 4, 2, 5, 3, 3, 2, 4, 4, 3, 1, ... (A090425(n)  1). E.g., for n=2 the height of 7 is 5 because it needs 5 iterations: 7 > 49 > 97 > 130 > 10 > 1.  R. J. Mathar, Jul 09 2017
ElSedy & Siksek prove that this sequence contains arbitrarily long subsequences of consecutive terms; that is, the upper uniform density of this sequence is 1.  Charles R Greathouse IV, Sep 12 2022


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. 319324, Arya Book Depot New Delhi 1996.


LINKS

E. ElSedy and S. Siksek, On happy numbers, Rocky Mountain J. Math. 30 (2000), 565570.


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 "8loop" which ends with (37,58,89,145,42,20,4,16,37) (End)


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. It enters a cycle 98 > 145 > 42 > 20 > 4 > 16 >37 >58 > 89 > 145 ...


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[n1]/10^k]10*Floor[a[n1]/10^(k+1)]) ^ (2) , {k, 0,
Floor[Log[10, a[n1]]] }]


PROG

(Haskell)
a007770 n = a007770_list !! (n1)
a007770_list = filter ((== 1) . a103369) [1..]
(PARI) ssd(n)=n=digits(n); sum(i=1, #n, n[i]^2)
(Python)
def ssd(n): return sum(int(d)**2 for d in str(n))
def ok(n):
while n not in [1, 4]: n = ssd(n) # iterate until fixed point or in cycle
return n==1
def aupto(n): return [k for k in range(1, n+1) if ok(k)]


CROSSREFS

Cf. A003132 (the underlying map), A001273, A035497 (happy primes), A046519, A031177, A002025, A050972, A050973, A074902, A103369, A035502, A068571, A072494, A124095, A219667, A239320 (base 3), A240849 (base 5).
Cf. A090425 (required iterations including start and end).


KEYWORD

nonn,base,nice,easy


AUTHOR



STATUS

approved



