## Time Subscript

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vahidi
Junior Member
Posts: 3
Joined: Fri Jan 05, 2018 1:10 am
Vensim version: DSS

### Time Subscript

Hi,
How can I formulate time subscript for variables?

For example, how can I define the formula in the attached pic?

In that formula p is variable.

Attachments
Interested formula
Untitled.png (3.17 KiB) Viewed 510 times

tomfid
Posts: 2425
Joined: Wed May 24, 2006 4:54 am

### Re: Time Subscript

This looks like discrete time notation, and presumably the summation indicates some kind of state variable dynamics. Hard to say how to implement it without knowing what it's for.

Generically, your two choices are to implement it directly using a subscript to represent the elements of j, or to use a stock to represent the dynamics explicitly.

However, since l(t-1) is independent of j in the summation, this simplifies readily:

r(t) = 1/p * p * l(t-1) = l(t-1)

http://metasd.com/2017/11/discrete-time-stinks/

vahidi
Junior Member
Posts: 3
Joined: Fri Jan 05, 2018 1:10 am
Vensim version: DSS

### Re: Time Subscript

Thanks tomfid,

The correct argument is l(t-j) as shown below:
Untitled.png (3.02 KiB) Viewed 494 times
I want to model a simple lending system as shown below:
Untitled1.png (52.73 KiB) Viewed 494 times
In this system, loans are made every year and have a redemption period (p). Although p is constant, I want to change it in different simulation runs.
Every year we must repay 1/p of the principal of each loan until full redemption. In fact, the r(t) is the amount of repayment each year that should be calculated by p and l.

vahidi
Junior Member
Posts: 3
Joined: Fri Jan 05, 2018 1:10 am
Vensim version: DSS

### Time Subscript

I want to model a simple lending system as shown below:
Untitled1.png (52.73 KiB) Viewed 475 times
In this system, loans are made every year and have a redemption period (p). Although p is constant, I want to change it in different simulation runs.
Every year we must repay 1/p of the principal of each loan until full redemption. In fact, the r(t) is the amount of repayment each year that should be calculated by p and l.

How can I formulate time subscript for variables?

For example, how can I define the formula in the attached pic for the loan redemption?
Untitled.png (3.02 KiB) Viewed 475 times

Posts: 3150
Joined: Wed Mar 05, 2003 3:10 am

### Re: Time Subscript

If I'm understanding correctly, all you need is

Code: Select all

``debt redemption = loan amount / loans redemption period``
http://www.ventanasystems.co.uk/forum/v ... f=2&t=4391

Units are important!
http://www.bbc.co.uk/news/magazine-27509559

tomfid