
Real Analysis Real Analysis Math Forum 
 LinkBack  Thread Tools  Display Modes 
August 11th, 2017, 10:51 AM  #1 
Newbie Joined: Aug 2017 From: san jose, ca Posts: 5 Thanks: 0  trying to find a name for this simple sequence?
n=1: 1 n=2: 1 + 2 n=3: 1 + 2 + 3 n=k: 1 + 2 + 3 + ... + k It has been long since I am in college and I looked at factorial and binomial only to find I grossly missed. Thanks. Last edited by skipjack; August 11th, 2017 at 01:03 PM. 
August 11th, 2017, 11:34 AM  #2 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,881 Thanks: 1088 Math Focus: Elementary mathematics and beyond 
They are 'triangular numbers'. They are of the form $\frac{n(n+1)}{2}$.

August 11th, 2017, 11:44 AM  #3 
Newbie Joined: Aug 2017 From: san jose, ca Posts: 5 Thanks: 0 
Thank you !

August 11th, 2017, 01:14 PM  #4 
Global Moderator Joined: Dec 2006 Posts: 19,865 Thanks: 1833 
Had you asked us to name this sequence: n=1: 1 n=2: 1 + 3 n=3: 1 + 3 + 5 n=k: 1 + 3 + 5 + ... + (2k1) we would have answered that these numbers are the positive perfect squares. Each sequence is a particular example of a quadratic sequence (because the value of the kth member of the sequence is given by a quadratic function of k). 
August 12th, 2017, 01:02 AM  #5 
Banned Camp Joined: Dec 2012 Posts: 1,028 Thanks: 24 
And, more in general the sequence for all Powers: 1^n=1 2^n=1+(2^n(21)^n) ... x^n=1+....+(x^n(x1)^n) or: $$A^n=\sum_{x=1}^{A}(x^n(x1)^n)$$ Known as telescoping sum properties. I use $x$ instead of "i" since all this becomes very interesting once sticked on a Cartesian Plane: Here the most simple case for $n=2$: And playing with the binomial develope / sum and fractions properties you can have back a Power of a Rational too. Here an example for n=3: Calling: $M_n= {[ x^n(x1)^n]}$ it becomes for $n=3$: $M_3= {( 3x^23x+1 )}$ Than: \[A^3 =\sum_{x=1}^{A}{( 3x^23x+1)} \] We can divide all the terms of our Sum by $K^3$ , remembering that if we want to left unchanged the result, we have to multiply the Upper limit by $K$ so we have: \[A^3 = \frac {K^3*A^3} {K^3} = \sum_{x=1}^{A*K}{( 3x^2/{K^3}3x/{K^3}+1/{K^3})} \] Now we can call: $ x= x/K $ , so changing $x$ with $x= x*K$, if we respect the following conditions: a) if and only if $K$ is a Factor of $A$, or perfectly divide $A$  the Upper limit becomes: $ K*A/K = A$ (with $K,A\in N*$ )  the Lower limit becomes: $ x=x/K$ so $x= 1$ becomes $x=1/K$ so: \[A^3 = \sum_{x=1/K}^{A}{( 3(x*K)^2/{K^3}3(x*K)/{K^3}+1/{K^3}) } \] Now we can simplify to have our new Step Sum, that moves Step $1/K$ from $1/K$ to $A$, so the Index $x$ will be $x =1/K, 2/K,3/K.... A$: \[A^3 = \sum_{x=1/K}^{A}{\left( \frac {3x^2}{K} \frac {3x}{K^2}+\frac{1}{K^3}\right)} \] for any $A\in \mathbb{Q}$ And pulling the Rational for $K\to\infty$ you discover the Integral: $$A^n=\int_{x=0}^{A} nx^{n1} dx$$ for any $A\in \mathbb{R}$ Before share this with your Prof. remember is not yet accepted by any math "authority", still if it was mine 10 years old trick Last edited by complicatemodulus; August 12th, 2017 at 01:08 AM. 

Tags 
find, sequence, simple 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Super simple sequence question  gazing600000  Algebra  15  November 13th, 2014 12:26 AM 
Sequence: find a2  Albert.Teng  Algebra  2  October 1st, 2012 09:16 AM 
find a pattern/sequence  westworld  New Users  5  April 15th, 2012 07:44 PM 
To find the sequence  Blickty  Algebra  6  September 24th, 2011 04:15 AM 
Find an example of a sequence with the given property  summerset353  Real Analysis  1  February 17th, 2010 05:11 PM 